Are you talking about a specific sound or distortion in the music itself?
A specific sound can be caused by any number of things - a damaged speaker,
a short in the speaker wiring, something rattling nearby, defective
electronics inside the sound card, defective electronics inside the speaker,
feedback through your microphone, and so on. It's nearly impossible for us
to know which without actually hearing the sound.
As for distortion, some amplified speakers for computers cannot handle the
power levels necessary for high volume levels. In an ideal system, you
should be able to turn the volume up to the level you prefer without turning
it up high enough to distort. If you want volume levels beyond that, you'll
need to purchase better speakers.
Dwight Stewart (W5NET)
http://www.qsl.net/w5net/