Audio volume fluctuating

Does anyone have any idea what might help me with the following issue:
During audio playback the volume of the track fluctuates from the normal volume to very quiet, and then back to the normal volume. This happens when using both winamp and windows media player so I assume it isn't anything to do with the application I use to play music.
I have tried reinstalling the audio drivers but the problem still persists.
I have an onboard (Intel DG33TL motherboard) Sygmatel sound chip, and am running Xp 64 bit.

Anyone have any ideas?

Is there anything that says Dynamic Range Control?

I cant see anything that says Dynamic Range Control, I've looked in the windows audio settings from the control panel and also in the audio control software that came bundled in with the audio driver.
Anywhere else worth checking?

Also when I play the same files on another computer or mp3 player they all play at the correct volume, so I guess it's not just dodgy files.

I have this problem but I think its actually my speakers as it occurred after
changing computer.

I don't think it is my speakers as when i plug them into my mp3 player or a different PC they play audio fine.
I have a cable from my PC connected to my amp, with sterio speakers attached. I have tried using the same speakers -> amp -> cable setup with the cable connected to different devices (laptop, mp3 player), and the PC is the only one to have this issue.
I too have only had this issue since I got a new computer, but the speakers still work fine with other devices...
It seems to turn the volume down during quiet parts of tracks, making it hard to hear, then when the volume on the track goes back up, the playback volume goes back to normal.

Anyone else have this problem or any suggestions? Thanks for the help!