The 7th Guest also messes with DOS's volume settings on some sort of internal level. I don't know enough about DOS to know what it's actually doing, but you can see a before and after here:
1) Before running the game. DOSBox's mixer settings show all volumes at 100%. I set FM to 0%.
2) After running the game. The game has now set FM back to 100% and set SB volume to 17%.
Wtf.