Have you only used the lime green port to connect your speakers?
A way back when, I tried 3 sets of speakers and they worked fine on my other PC but not on this one that I'm now using.
Eventually out of desperation I connected to the light blue port and that cured my problem. The PC that I'm using to write this is still connected that way(just checked). I don't know why but this PC just works that way.
This is an older system (Intel Pentium D 820) and I'm running Vista. Sound device NVIDIA High Definition Audio.
If you haven't tried this give it a shot. overthehill