Unfortunately I haven't documented this yet. However, the current code has been completely rewritten and has been designed to support multiple operating systems and sound systems.
For details of the internal interface and any available patches see my WWW page at http://www.slitesys.demon.co.uk/a.macdonald/dosemu/sound/
At a later stages the code was extensively reworked (again) by Stas Sergeev. Instead of direct pipes connected to /dev/dsp the DMA code now uses callbacks. If pipes will be advantageous for other types of DMA in the future, they will be readded. Opinions differ on this very subject (whether pipes are more flexible than callbacks in certain cases).
Copyright (C) 1995 Joel N. Weber II This sound code is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
The mixer emulator probably doesn't do the math precisely enough (why did Hannu use a round base 10 number when all the sound boards I know use base 2?) It ignores obscure registers. So repeatedly reading and writing data will zero the volume. If you want to fix this, send Hannu a message indicating that you want the volume to be out of 255. It will probably fix the problem that he advertises with his driver: if you read the volume and write it back as is, you'll get zero volume in the end. And on the obscure registers issue, it only pays attention to volumes and the dsp stereo bit. The filters probably don't matter much, but the record source will need to be added some day.
The dsp only supports reset and your basic dma playpack. Recording hasn't been implemented, directly reading and writing samples won't work too well because it's too timing sensitive, and compression isn't supported.
FM currently has been ignored. Maybe there's a PCGPE newer than 1.0 which describes the OPL3. But I have an OPL3, and it would be nice if it emulated that.
MIDI and joystick functions are ignored. And I think that DOSEMU is supposed to already have good CDROM support, but I don't know how well audio CD players will work.
If you're having performance problems with wavesample playback, read /usr/src/linux/drivers/sound/Readme.v30, which describes the fragment parameter which you currently can adjust in ../include/sound.h
I haven't tested this code extensively. All the software that came with my system is for Windows. (My father claimed that one of Compaq's advantages is user friendlyness. Well, user friendlyness and hackerfriendlyness don't exactly go hand in hand. I also haven't found a way to disable bios shadowing or even know if it's happening...) I can't get either of my DOS games to work, either (Descent and Sim City 2000). Can't you guys support the Cirrus? (Oh, and while I'm complaining, those mystery ports that SimEarth needs are for the FM synthesiser. Watch it guys, you might generate interrupts with that....)