Is there anything nearby that may be causing interference with the speakers? Wires, printers, etc? If so, try to move things around and space them out a bit.
You should also test out the sound by plugging headphones into your computer. If the sound works fine then you know the issue is with the speakers themselves and not your system.
Another option is to try turning down the inputs on the sound card a little bit if they are on 100% and see if that reduces the noise any.