I am running the following.
SONY Bravia KDL-37w5500
SONY DAVD-Z660 Surround Sound System
I run an optical cable to the surround from the TV
I run HDMI Leads from both xbox & surround to the TV
My old 360 used to send surround sound via HDMI.
I only get 5.1 when plugging the optical cable directly into the surround sound from the XBOX
Hugely annoying as then I get no TV volume when I turn the XBOX off!!!
Constantly having toi fiddle swapping the optical cables over!!!!!!!!!!