Do you have an HDTV? There's two options you could do. If you have an hdtv, plug the blue, green, and red cables into the tv and plug only red and white audio cables (they have a speaker icon imprinted) into your theatre. If your home theatre supports optical sound, it's even easier. Go ahead and plug the same three blue, green, and red into the tv. Then you'll need to purchase an optical cable. One end will go into your home theatre. The other end will go into the xbox 360 video cable. Hope that helps.
The new xbox 360 does not even get blu ray , only for that i bought a ps3 1 month ago. Also more than 50% owners get the red ring of death.
Just plug the audio(red and white) into the auxilliary of the home theater system.. You wont get picture from it just audio. Plug the yellow into the tv. Your sound will come from the home theater system.
It's simple . you already have a picture and audio to tv via hdmi. connect the red and white audio out from xbox to an open audio input on the receiver and select that input on the receiver.