OK, well take the stereo jack out of the computer, and plug it into a Walkman, and see what you get. This will confirm if it's a problem with the speakers or the sound card.
If your left/right speakers are interchangeable (mine are not), reverse the connections. If the sound then moves to the left, you have a problem with the individual speaker that was on the left. If not, then the problem is with the sub passing the signals out to the speakers.
Try turning the volume all the way up, with no input signal. Are all the speakers hissing? If not, are none of them hissing? If only the speaker that is not playing sound is not hissing, then the chances are it's a broken wire/pin in the plug.