I just had a similar problem with my front left speaker. In my system, a cable connects from sound card jacks to front right speaker (with controls), then another cable goes from there to the subwoofer. From there, there are separate output jacks to the other 4 speakers. So I plugged the line to the left front speaker into another jack and found that the speaker itself was dead - not the signals going to it. Verified with an ohm meter. Opened up the speaker case to investigate and trace continuity with the meter, and discovered a solder joint inside that had never been soldered when originally assembled. It had worked OK when new, but shook loose later! A quick solder job and all is good.