6.1 Receiver In 2009 Help With Onkyo HT-R520 Reciever And Xbox.?

Help with onkyo HT-R520 reciever and Xbox.? - 6.1 receiver in 2009

I have is worth a Onkyo HT-R520 6.1 receiver and my Xbox 360 to connect an optical cable. My problem is that it plays in stereo with sub. I checked all connections and are good. I also play my settings on my Xbox to Dolby Digital. any help would be appreciated.

Thank you in advance.