Have you been able to get 5.1 sound out of another source such as a DVD played through the pc? Just because you have 5 speakers doesn't mean they'll be used by every source. The "multi ch in" means that the source is sending multiple sound channels to your Denon. So that could mean that your getting 2 channels with sound and 3 without. Nothing is wrong in that case, just means the source doesn't have surround sound.