Are you using shielded audio cable throughout? Check that you have good contact between the spring loaded contacts in the female speaker plugs on the computer and the male plug on your speaker cord. Try using first the rear, then front, speaker plugs on your computer if available. Try relocating the speakers and speaker wiring or the computer itself to whatever extent may be possible. Chances are good that you simply have a poor connection somewhere or have some unshielded audio cable. Weird as it may sound, you may find improvement by running the speaker wires through a section of metal conduit (which would act as a shield) and grounding the conduit to a safe earth ground through your home electrical system or an electrically conductive plumbing system (iron or copper piping). The material used for this experiment could be any kind of metal tubing and does not need to be specifically electrical conduit. Aluminum or even copper tubing works as well as any other electrically conductive material for this purpose. Providing a solid path to ground is the important thing here. Make sure your computer power cord is connected to a properly grounded three wire circuit.
Keep working with it, it is a solvable problem.
Good Luck, John