Sigmatel sound problem

I am using the onboard sound on an Intel 955XCS with Inspire 5300 speakers. My previous rig had an Audigy 2, and the speakers worked flawlessly. On this rig, I cannot get sound from the rear speakers under normal circumstances. I tested the output with headphones and get the same results. When I open Intel Audio Studio and test each speaker set they work, but playback of music or DVD or the full speaker test music sends nothing to the rear or center. I have emailed Intel support, they had me uninstall-reinstall the software, play with the cmos etc., but it seems to be within whatever decodes the music, because I get sound on the test hiss or whatever you call that tone. Suggestions appreciated. Thank you.