5.1 sound with Inspiron 1420

I've got a 5.1 speaker system with a green cable, an orange cable, and a black cable. The laptop has dual headphone jacks and a mic-in jack. When I open SigmaTel Audio from the control panel, it allows me to set the jacks to 5.1 sound, with the left headphone jack green (front speakers), the middle jack orange (center speaker), and the right black (rear speakers, sub too?). However, the mic jack doesn't send anything to the rear speakers, and I don't really notice the sub going at all, which is why I suspect the sub's supposed to be controlled by the black jack. When I unplug the green and orange jacks, the sound routes back to the computer speakers, even though the SigmaTel panel still shows the setup as 5.1 audio with one jack plugged in.

My question is: is this an inherent discrepancy between the capabilities of the hardware and the software, or is there a way to get this to work? Right now I'm running all three jacks through two daisy-chained splitters into one headphone jack, and the speakers are dying to be released from this two-channel weakness into the realm of true surround sound. Any way to get this to work as it is before I drop some cash on an external sound solution?

 

Bump... I don't know if that's really considered kosher here, but I can't find help anywhere, and this issue is really bothering me... as an update, the middle channel doesn't seem to be producing either, I don't know if it was working at first or not, I thought it was, but now I'm not sure, since it's not working now...