When I play a movie in Dolby 5.1 I only hear sound coming from the left and right front speakers, center and subwoofer. The back sorrounds seem to be muted. However, when i play a DTS track, it fully utilizes all 5 speakers. Anyone know what's going on? It's only been doing this for the past week or so. I purchased the unit back in December. Any ideas or thoughts would be greatly appreciated.