I've just bought a Polk AMR130 6-piece computer speaker system. I've hooked it up to my Turtle Beach Santa Cruz sound card, which has 5.1 capability. I've double-checked my connections, and they all appear to be correct. The front and center speakers and subwoofer are working to perfection--BUT--I'm not getting ANY sound out of the rear channels at all! I've double-checked my sound card's software control settings, and they all appear to be correct. I know that the jack for the rear channels is working, because I had my stereo headphones plugged into it (as the manual specified for headphones), and it was working the last time I checked. Any suggestions on how to proceed or what to check next? Thanks in advance.