The Receiver will have a bunch of input jacks...red and white. It should also have a bunch of optical inputs, they're little black holes with a shutter door or plug. You should see them labeled as CD, DVD, TV, etc. Let's say you're using RCA. Connect the red and white cables to the TV audio out red and white. Connect the other end to the TV input red and white. Set the receiver to TV and you should have sound
Optical is better, if you have it on the receiver. They're usually in a line down the left side of the receiver, if you're looking at it from the back. If you have optical, use it instead of the rca's.
Your other option is to run the sound directly to the receiver from the xbox and bypass the tv.