First, try to find a disc with some test tones (the "THX Optimizer" feature on some DVD's should work for this, as should Digital Video Essentials or AVIA) and use those test tones. Next, start swapping cables and making sure all the cables are plugged in correctly. I've had cases where one end or the other was just loose enough on several cables that I had problems like this. As XenonMan suggests, also try using the right front and left surround cables with other channels. The goal is to determine if any cables are bad or if the player's outputs are at fault.