I having trouble with my xbox 360?

I have a xbox 360 connected to a Sony STR-DG500 and the sound is screwed up. I have it connected by a ditigal cable. I have 5 speaker and now it is only using 3 of them. the 2 back and 1 main front. And the main amp. It not using the side front speaker 2 speakers at all. And it is doing it with my pc to. Help