This issue seems to have cropped up recently, though I had it before and managed to fix it. I upgrade my video card to an nVidia GeForce 9500 GE, which recommended more power than my stock power supply could produce. I thought I'd ignore it, but I noticed that playing music and doing any sort of work would cause the sound to distort and skip.
I upgraded the power supply to a newer one that was well above the recommended wattage for the video card, and the sound issue went away.
Recently though, it's been happening again and I'm not sure why. It only seems to happen if I'm browsing or playing a game, but just letting the music play causes no problems.
Does anyone have an idea as to what causes this distortion? Is there anything I can do?