It could be speaker interference. Try moving the computer away from them, and position them away from other electronics. Attempt to silence (turn off) other electronics in the area just as a test. Say you have a mini-refrigerator nearby: its power draw can interfer with the speakers. After all this, make sure you have the most recent audio drivers
installed for your soundcard/motherboard. Make sure they are plugged in well enough.
Attempt to plug the speakers into another audio jack on your computer. I think computer fans interfered with my last audio setup.