If for some reason you don't want to install the OpenAL files with oailnst.exe, you can do the following:
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Download the openal-soft-x.xx-bin.zip (where x.xx is the version number).
From the Win32 directory in the archive, extract soft_oal.dll to the ManicDigger directory.
Rename soft_oal.dll to OpenAL32.dll
Start the game and enjoy.