Well, a sound card won't hurt any! It'll do you good, but I don't think that this will fix the problem.
Have you tried relocating your speakers? Move them to a different spot, and make sure that they're not near any power cords, or any type of cords for that matter. The speaker wire themselves might be picking up interference from a power cord or another cord. Cordless phones have been known to interfere with some speakers (only happened to me once though). You might have to move your speakers more than once, and don't forget to move your subwoofer.
Hope this helps! Let us know how you made out with this.
War produces veterans, wounded both physically and mentally. They have sacrificed for us.....and it is now our job to help these veterans, as they have already helped us in ways we will never know, in ways that we cannot fathom, and in ways that we take granted every day.