there are too many permutations to go into here but the jest is this:

Depending on the codecs and where the decoding is actually happening, TV's can sometimes downgrade audio that is being sent out to a receivr. It could be something in the settings of the TV, or the Xbox or the receiver.

The ideal way to handle surround sound is to send all audio in bitstream form via HDMI to a receiver to let it do the decoding internally. The video then goes out via HDMI to the TV. Since your Receiver has no HDMI inputs you will have to go through the TV.

The only advice I can really offer is research the limitations of HDMI and optical cables and go through the settings on all your components to make sure they are treating the signal the way you want it to.

Also, be aware that the Xbox 360 has a known HDMI glitch where even though games are available in DD 5.1 it will send the signal out over HDMI as stereo. Sometimes you have to go in and reset it to DD 5.1 several times in a row as it keeps reverting to stereo.