dolby 5.1 question using comcast box
I have a 7.1 surround system with a sony strd 790. I have the cable box via hdmi to the receiver. When I choose 5.1 in the comcast menu my center channel does not produce any sound. Everything else works. The center works with the dvd that is also routed through my recevier. When hooked up to the cable box the receiver does recognize dolby surround and it displays that it is receiving the signal through the menu. In stereo mode through comcast I get great sound, but it is not 5.1. I have tested the receiver speakers and they are all working normally. I'm not sure how to fix this. thanks.