Advanced Linux Sound Architecture (ALSA) project homepage
Alsa Opensrc Org

Using ffmpeg (for most audio formats):

SDL_AUDIODRIVER="alsa" AUDIODEV="hw:0,0" ffplay -nodisp audiofile.flac

Source:Super User - select ffplay audio out device

Using mpg123 (for mp3's only):

mpg123 -a hw:1,0 audiofile.mp3

For example, to dump the hardware parameters for a Digidesign MBOX 2:

arecord --dump-hw-params -D hw:M2,0

Recording WAVE 'stdin' : Unsigned 8 bit, Rate 8000 Hz, Mono
HW Params of device "hw:M2,0":
--------------------
ACCESS:  MMAP_INTERLEAVED RW_INTERLEAVED
FORMAT:  S24_3BE
SUBFORMAT:  STD
SAMPLE_BITS: 24
FRAME_BITS: 48
CHANNELS: 2
RATE: 48000
PERIOD_TIME: [1000 1820438)
PERIOD_SIZE: [48 87381]
PERIOD_BYTES: [288 524286]
PERIODS: [2 1024]
BUFFER_TIME: [2000 3640875]
BUFFER_SIZE: [96 174762]
BUFFER_BYTES: [576 1048572]
TICK_TIME: ALL
--------------------
arecord: set_params:1297: Sample format non available
Available formats:
- S24_3BE

Here you can see the MBOX2 only supports the S24_3BE (signed 24-bit 3bytes Big Endian) format.

Source:SourceForge - ALSA - Mailing Lists - Re: [Alsa-user] arecord: begin_wave:2480: Wave doesn't support S24_3BE format

Get a list of midi devices:

aplaymidi -l

Example output:

 Port    Client name                      Port name
 14:0    Midi Through                     Midi Through Port-0
 20:0    USB Midi                         USB Midi MIDI 1
 44:0    Mbox 2                           Mbox 2 MIDI 1
 48:0    Mbox 2                           Mbox 2 MIDI 1

To use the midi device named “USB Midi” on port 20:0 in above output in DOSbox, edit the [midi] part in your DOSbox config in ~/.dosbox/dosbox-VERSION.conf (in my case ~/.dosbox/dosbox-0.74-3.conf) to look like this:

[midi]
#     mpu401: Type of MPU-401 to emulate.
#             Possible values: intelligent, uart, none.
# mididevice: Device that will receive the MIDI data from MPU-401.
#             Possible values: default, win32, alsa, oss, coreaudio, coremidi, none.
# midiconfig: Special configuration options for the device driver. This is usually the id of the device you wan
t to use.
#               See the README/Manual for more details.

mpu401=intelligent
mididevice=default
midiconfig=20:0

Show all soundcards that are recognised by ALSA:

aplay -l | grep card

Test playback on a specific card:

speaker-test -c2 -twav -Dhw:Creative

Test playback on a specific pcm or dmix device definied in a manually edited .asoundrc:

speaker-test -c2 -twav -Ddmixsoundblasterz

This helps to track down vague error messages in a .asoundrc that sends sound output to multiple soundcards simultaneously.

For faking a 7.1 channel setup on a stereo soundcard.

Save the below code to ~/.asoundrc and change the pcm.maudiomicrodac part to match your card (see 'aplay -l | grep card' output):

pcm.!default {
        type asym
        playback.pcm "plug:stereo2quad"
}

pcm.dmixer  {
        type dmix
        ipc_key 1024
        slave {
                pcm "maudiomicrodac"
                period_time 0
                period_size 1024
                buffer_size 4096
                rate 48000
        }
        bindings {
                0 0
                1 1
                2 0
                3 1
                4 0
                5 1
                6 0
                7 1
        }
}

pcm.stereo2quad {
        type route
        slave.pcm "dmixer"
        ttable.0.0 1
        ttable.1.1 1
        ttable.2.2 1
        ttable.3.3 1
        ttable.4.4 1
        ttable.4.5 1
        ttable.5.4 1
        ttable.5.5 1
        ttable.6.6 1
        ttable.7.7 1
}

pcm.maudiomicrodac {
        type hw
        card DAC
        device 0
        channels 2
}

ctl.dmixer {
        type hw
        card 0
}

Sources:

• bootlin - Audio multi-channel routing and mixing using alsalib
• unix stackexchange - Splitting the outputs of a 7.1 USB sound card into 4 distinct stereo pairs
• askubuntu - How to split 7.1 soundcard outputs to 4 stereo outs with ALSA
• AlsaProject - Asoundrc
• AlsaProject - PCM (digital audio) plugins PCM plugins extends functionality and features of PCM devices. The plugins take care about various sample conversions, sample copying among channels and so on.

Send to both onboard and extra HDMI port (HDMI and HDMI_1):

Save the below code to ~/.asoundrc

pcm.!default {
        type asym
        playback.pcm "plug:stereo2quad"
}                                         

pcm.multi {
        type multi;
        slaves.a.pcm dmixHDMI
        slaves.b.pcm dmixHDMI_1
        slaves.a.channels 2;
        slaves.b.channels 2;
        bindings.0 { slave a; channel 0; }
        bindings.1 { slave a; channel 1; }
        bindings.2 { slave b; channel 0; }
        bindings.3 { slave b; channel 1; }
}

pcm.stereo2quad {
        type route
        slave.pcm "multi"
        ttable.0.0 1
        ttable.1.1 1
        ttable.0.2 1
        ttable.1.3 1
}

pcm.dmixHDMI  {
        type dmix
        ipc_key 1024
        slave {
                pcm "HDMI"
                period_time 0
                period_size 1024
                buffer_size 4096
                rate 48000
        }
}

pcm.dmixHDMI_1  {
        type dmix
        ipc_key 1025
        slave {
                pcm "HDMI_1"
                period_time 0
                period_size 1024
                buffer_size 4096
                rate 48000
        }
}

pcm.HDMI {
        type hw
        card HDMI
        device 7
        channels 2
}

pcm.HDMI_1 {
        type hw
        card HDMI_1
        device 7
        channels 2
}

ctl.dmixer {
        type hw
        card HDMI
}

Example of an ~/.asoundrc with even more soundcards, some of them commented out:

pcm.!default {
	type asym
	playback.pcm "plug:stereo2quad"
}

	#capture.pcm "plug:dsnoop:CODEC"
pcm.multi {
	type multi;
	slaves.a.pcm dmixbehringeruca202
	slaves.b.pcm dmixsennheisergsx1200pro
	slaves.c.pcm soundblasteraudigyrx
	slaves.d.pcm dmixsoundblasterz
	slaves.e.pcm dmixfasttrack_1
	slaves.f.pcm dmixonboard
	slaves.g.pcm dmixtascamus100
	slaves.h.pcm dmixsoundblasterrecon3dusb
	slaves.i.pcm dmixmbox3mini
	slaves.j.pcm dmixbehringeruca200
	slaves.k.pcm soundblasterxfi
	slaves.l.pcm plugmbox2_1
	slaves.m.pcm plugmbox2_2
	slaves.a.channels 2;
	slaves.b.channels 2;
	slaves.c.channels 2;
	slaves.d.channels 2;
	slaves.e.channels 2;
	slaves.f.channels 2;
	slaves.g.channels 2;
	slaves.h.channels 2;
	slaves.i.channels 2;
	slaves.j.channels 2;
	slaves.k.channels 2;
	slaves.l.channels 2;
	slaves.m.channels 2;
	bindings.0 { slave a; channel 0; }
	bindings.1 { slave a; channel 1; }
	bindings.2 { slave b; channel 0; }
	bindings.3 { slave b; channel 1; }
	bindings.4 { slave c; channel 0; }
	bindings.5 { slave c; channel 1; }
	bindings.6 { slave d; channel 0; }
	bindings.7 { slave d; channel 1; }
	bindings.8 { slave e; channel 0; }
	bindings.9 { slave e; channel 1; }
	bindings.10 { slave f; channel 0; }
	bindings.11 { slave f; channel 1; }
	bindings.12 { slave g; channel 0; }
	bindings.13 { slave g; channel 1; }
	bindings.14 { slave h; channel 0; }
	bindings.15 { slave h; channel 1; }
	bindings.16 { slave i; channel 0; }
	bindings.17 { slave i; channel 1; }
	bindings.18 { slave j; channel 0; }
	bindings.19 { slave j; channel 1; }
	bindings.20 { slave k; channel 0; }
	bindings.21 { slave k; channel 1; }
	bindings.22 { slave l; channel 0; }
	bindings.23 { slave l; channel 1; }
	bindings.24 { slave m; channel 0; }
	bindings.25 { slave m; channel 1; }
}

pcm.stereo2quad {
	type route
	slave.pcm "multi"
	ttable.0.0 1
	ttable.1.1 1
	ttable.0.2 1
	ttable.1.3 1
	ttable.0.4 1
	ttable.1.5 1
	ttable.0.6 1
	ttable.1.7 1
	ttable.0.8 1
	ttable.1.9 1
	ttable.0.10 1
	ttable.1.11 1
	ttable.0.12 1
	ttable.1.13 1
	ttable.0.14 1
	ttable.1.15 1
	ttable.0.16 1
	ttable.1.17 1
	ttable.0.18 1
	ttable.1.19 1
	ttable.0.20 1
	ttable.1.21 1
	ttable.0.22 1
	ttable.1.23 1
	ttable.0.24 1
	ttable.1.25 1
}

pcm.plugmbox1 {
	type plug
	slave.pcm "dmixmbox1"
}

pcm.plugmbox2_1 {
	type plug
	slave.pcm "dmixmbox2_1"
}

pcm.plugmbox2_2 {
	type plug
	slave.pcm "dmixmbox2_2"
}

pcm.dmixbehringeruca202  {
 	type dmix
 	ipc_key 1024
 	slave {
		pcm "behringeruca202"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsennheisergsx1200pro  {
        type dmix
        ipc_key 1025
        slave {
                pcm "sennheisergsx1200pro"
                period_time 0
                period_size 1024
                buffer_size 4096
		rate 48000
       }
	bindings {
		0 0
		1 1
#		2 2
#		3 3
#		4 4
#		5 5
#		6 6
#		7 7
	}
}

pcm.dmixhyperx  {
 	type dmix
 	ipc_key 1026
 	slave {
		pcm "hyperx"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixhyperxamp  {
 	type dmix
 	ipc_key 1027
 	slave {
		pcm "hyperxamp"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}


pcm.dmixonboard  {
 	type dmix
 	ipc_key 1028
 	slave {
		pcm "onboard"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsoundblasterz  {
 	type dmix
 	ipc_key 1029
 	slave {
		pcm "soundblasterz"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsoundblasterzxr  {
 	type dmix
 	ipc_key 1030
 	slave {
		pcm "soundblasterzxr"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsoundblasterrecon3dusb  {
 	type dmix
 	ipc_key 1031
 	slave {
		pcm "soundblasterrecon3dusb"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

#pcm.dmixturtlebeachearforce450  {
# 	type dmix
# 	ipc_key 1032
# 	slave {
#		pcm "turtlebeachearforce450"
#		period_time 0
#		period_size 1024
#		buffer_size 4096
#		rate 48000
#	}
#	bindings {
#		0 0
#		1 1
#	}
#}

#pcm.dmixmixamp  {
# 	type dmix
# 	ipc_key 1033
# 	slave {
#		pcm "mixamp"
#		period_time 0
#		period_size 1024
#		buffer_size 4096
#		rate 48000
#	}
#	bindings {
#		0 0
#		1 1
#	}
#
#}

pcm.dmixbehringeruca200  {
 	type dmix
 	ipc_key 1034
 	slave {
		pcm "behringeruca200"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}

}

#pcm.dmixstrixraiddlx  {
# 	type dmix
# 	ipc_key 1035
# 	slave {
#		pcm "strixraiddlx"
#		period_time 0
#		period_size 1024
#		buffer_size 4096
#		rate 48000
#	}
#	bindings {
#		0 0
#		1 1
#	}
#}

pcm.dmixmbox3mini  {
 	type dmix
 	ipc_key 1036
 	slave {
		pcm "mbox3mini"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

# 2019-06-09
# Added dmix device despite hwmix support as
# periodically I still got "Device or resource busy".
# But the dmix device didn't help...
#pcm.dmixsoundblasterxfi  {
# 	type dmix
# 	ipc_key 1037
# 	slave {
#		pcm "soundblasterxfi"
#		period_time 0
#		period_size 1024
#		buffer_size 4096
#		rate 48000
#	}
#	bindings {
#		0 0
#		1 1
#	}
#}

pcm.dmixmbox2_1  {
 	type dmix
 	ipc_key 1038
 	slave {
		pcm "mbox2_1"
		period_time 0
		period_size 1024
		buffer_size 4096
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixmbox1  {
 	type dmix
 	ipc_key 1039
 	slave {
		pcm "mbox1"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixfasttrack_1  {
 	type dmix
 	ipc_key 1040
 	slave {
		pcm "fasttrack_1"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixfasttrack_2  {
 	type dmix
 	ipc_key 1041
 	slave {
		pcm "fasttrack_2"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}


pcm.dmixmbox2_2  {
 	type dmix
 	ipc_key 1042
 	slave {
		pcm "mbox2_2"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsoundblastertactic3drageusb { 
 	type dmix
 	ipc_key 1043
 	slave {
		pcm "soundblastertactic3drageusb"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixsoundblasteraudigyrx { 
 	type dmix
 	ipc_key 1044
 	slave {
		pcm "soundblasteraudigyrx"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.dmixtascamus100 { 
 	type dmix
 	ipc_key 1045
 	slave {
		pcm "tascamus100"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 48000
	}
	bindings {
		0 0
		1 1
	}
}

pcm.behringeruca202 {
	type hw
	card CODEC
	device 0
	channels 2
}

pcm.behringeruca200 {
	type hw
	card CODEC_1
	device 0
	channels 2
}

pcm.fasttrack_1 {
	type hw
	card Track
	device 0
	channels 2
}

pcm.fasttrack_2 {
	type hw
	card Track_1
	device 0
	channels 2
}

pcm.hyperx {
	type hw
	card Sound
	device 0
	channels 2
}

pcm.hyperxamp {
	type hw
	card Amp
	device 0
	channels 2
}

#pcm.mixamp {
#	type hw
#	card MixAmp
#	device 0
#	channels 2
#}

pcm.mbox1 {
	type hw
	card MBox
	device 0
	channels 2
	format S24_3BE
	rate 48000
}


pcm.mbox2_1 {
	type hw
	card M2
	device 0
	channels 2
	format S24_3BE
}

pcm.mbox2_2 {
	type hw
	card M2_1
	device 0
	channels 2
	format S24_3BE
}

pcm.mbox3mini {
	type hw
	card Mini
	device 0
	channels 2
}

pcm.onboard {
	type hw
	card Generic
	device 0
	channels 2
}

pcm.sennheisergsx1200pro {
        type hw
        card Audio
        device 1
        channels 2
}

pcm.soundblasteraudigyrx {
	type hw
	card Audigy2
	device 0
	channels 2
}

pcm.soundblasterrecon3dusb {
	type hw
	card Recon3D
	device 0
	channels 2
}

pcm.soundblastertactic3drageusb {
	type hw
	card USB
	device 0
	channels 2
}

pcm.soundblasterxfi {
	type hw
	card XFi
	device 0
	channels 2
}

pcm.soundblasterzxr {
	type hw
	card Creative_1
	device 0
	channels 2
}

pcm.soundblasterz {
	type hw
	card Creative
	device 0
	channels 2
}

#pcm.strixraiddlx {
#	type hw
#	card DLX
#	device 0
#	channels 2
#}

pcm.tascamus100 {
	type hw
	card CODEC_2
	device 0
	channels 2
}

#pcm.turtlebeachearforce450 {
#	type hw
#	card E450
#	device 0
#	channels 2
#}

ctl.dmixer {
	type hw
	card 1
}

Sources:

• Slackware 13.37 for dummies - Alsa: multiple output, multiple sound cards, multiple users
• 6by9 - Output to multiple audio devices with alsa
• stackoverflow - ALSA how to duplicate a stream on 2 outputs and save system configs