If the speakers are unplugged from the computer and no sound can now go through them, how does the speaker "buzz"?
That aside, normal fixes for speaker buzzes are the same whether with computers or with other sound systems.. If you can repair the buzzing portion of the speaker, such as a torn cover/screen, dust in the speaker cones, or bad connection to the speaker box, do so.. If you can't repair such, replace the speaker.
Hope this helps.
A few months ago I bought a set of Sony SRS D21 computer speakers. At first there were no problems, but a few days ago I noticed a soft (and annoying!!!) buzzing sound coming from the right speaker. When I increase or decreace the volume of the speakers or the computer it has no effect. The music volume changes, but the buzzing remains the same, so when I play music softly, I hear the buzz coming through. When I unplug the speakers from the computer the buzz is still there. I tried pluggung the power plug into a different wall socket, but that didn't help either. What could be causing this? I know it can't be my sound card. And if it is some kind of interference, why is it only effecting the right speaker?