PC version sound problems

[GrlGmr]

I'm only on the first level after the tutorial, but a problem has started to rear its ugly head: electronic buzzing, like feedback. I know it's the game, because I can alt+tab out and listen to other things just fine without touching the speakers, so it's not my speakers. As soon as I alt+tab in, the buzzing starts up, but I can listen to MP3s right after tabbing out. When I start the level or restore a previous save, the problem goes away, but reappears a few minutes later. It seems to start quietly, and gets worse and worse, until you can barely understand the voices. This is the fist game I've ever had this problem on.

Here is my setup:

Asus A7N8X mobo (it's an nForce 2 nVidia board)
Generic Creative PC speakers
Using the mobo's onboard sound
Athlon XP 3200+
512MB DDR RAM
Radeon 9800 PRO

[Dash]

1. Don't alt tab
2. Turn off hardware mixing.
3. Check power supply