Auto muting

My pc keeps muting itself, especially while I am gaming. After a minute or two of playing any game, my pc will mute the wave volume control all on its' own. Very annoying. I am using winxp. I've never had a problem quite like this one. If I unmute it, then it will just mute itself again. It will only do this when I am online. I have checked for virus and adware, etc. Before you ask, I have loads of virus and adware scanners, although, I suppose anything's possible.

Anyone seen anything like this before?

:confused