Check if there are any other electronic devices in the vicinity. Speakers sometimes tend to pick up interference from the surroundings, and having an appliance in the vicinity doesn’t help much. Try moving your speakers and wires around and see if it helps. A common culprit is the radio, and should be the first thing to be removed from the immediate surroundings. Your speakers shouldn’t be too close to your monitor, either.
Another cause of your problem could be because of how your speakers are placed. Subwoofers are best under your table—for one thing, they sound best from there; secondly, they aren’t magnetical-ly shielded, so having them closer to your monitor can cause distortions in both sound and picture. If you own a 5.1 surround sound speaker system, keep in mind that audio from the centre speaker fans out horizontally, so the best place for it is above your monitor, where the sound won’t encounter any obstacles. Finally, check the speakers themselves—plug them into your music system as well, and check if they work fine. If they do, then the problem goes deeper than just speakers.
To get the most out of your subwoofer, place it in a corner. The sound will reflect off the three wall surfaces, sending a very desirable blast all across your room.