Is the sound driver anything to do with VIA? I had terrible trouble with my old laptop along exactly the same lines as you describe. The only way to solve it was to either reinstall the drivers from the CD which came with the machine or to go back to a previous restore point which sometimes did the trick. In the end it just wouldn't work at all and I bought a macbook. I don't seem to have any of the problems I used to have with windows. One other thing, have you found that the little volume icon on your toolbar has disappeared? That used to be a symptom.
I hope you find a solution.
TN