AUDIO: Register MT-32 emulator as plain sound type.

Formerly the audio stream was registered as sfx. This is incorrect behavior
since the client code will control music volume with MIDI events on its own.
It seems 67b311713d8f4cfcd460a9649e0075f24278a048 introduced this very long
ago.

This should fix unintended coupling of sfx volume and music volume in BASS.
AUDIO: Register FluidSynth driver as plain sound type.

MIDI code will control volume via MIDI events thus the generated audio should
not be affected by mixer sound volumes.

The initial commit(s) in d4d045b1174b4a48659f39f026ade42684b679bf /
13dc149ded691e718905049990dd0220230c500e added it as music sound type.
So, this seems to be a long standing issue.
AUDIO: Register EAS sound driver as plain sound type.

MIDI code will control volume via MIDI events thus the generated audio should
not be affected by mixer sound volumes.

The initial commit 0e6cdfd67580f75e912c5e92abb26821d032f74b added it as music
sound type. Might be copied from the (also) incorrect FluidSynth code.