I'm kind of confused. But you do know that the 360 DOES NOT output sound via HDMI, correct?
It's very misleading from MS, but you have to have a separate audio connection if you are using HMDI. Yes, it pretty much defeats the purpose, as really the only pro's to using HDMI on the xbox is very minimal PQ improvement, 1080p where available, and for bragging.
You need to use optical+HDMI... you can't just do HDMI. Personally, since I have a 720p television, component+optical does great for me.