I had a driver for Win95 that could do midi wavetable through PC speaker. It sounded pretty incredible, but I remember it locking up the whole system to play a song. Didn't seem to be CPU bound either. Felt almost like an interrupt thing. Definitely made games unplayable, and was only good for listening to midi files. Likewise I don't really recall any high framerate DOS games doing PC speaker audio. I'm assuming there's a reason for that.