I had this same problem. You need to get one of those optical audio adapters that comes with the official 360 hdmi cable. First of all you will need it because the a/v plug is tooooo damn fat and blocks the hdmi port, which I'm sure you already noticed. I custom made my adapter but you can pay $50 and and use the one that comes with 360 hdmi.
On a side note, to get sound from your receiver and not through the tv, you have to plug in the audio cable first and let the xbox turn on. Once it's on, then plug in your hdmi cable. That way the hdmi will only be used for video. How do I know? I almost threw my 360 out the window until I finally figured it out.
Xbox.com is limited on info about this.