Usually, when I've seen this happen, it's because:

- You have the volume sliders in the Sound Mixer applet turned way way down, and
- You have the volume knob on the speakers themselves turned way way up to compensate.

The result is that you are amplifying the noise floor of your sound card. This is very similar to having the gains on the amps in your car improperly adjusted.

It's also possible that it's a ground loop of some kind, or even a bug in the mouse driver, video drivers, or audio drivers. But the first thing I'd check is your volume levels.