What I mean by the HD/SD feed is that channel 13 is the SD feed of CBS, then if I go to channel 13-1 I'm now watching the HD feed of CBS. That is when I watch 13 I have sound, but 13-1 I don't.
As for things connected to the TV, it's just the antenna and my XBOX360 via component.
I poked around in all the audio options and it's all set to go through the speakers, not receiver.
Now, I'm obviously in over my head, but do I need to connect something to the "Digital audio out" jack on the back of the TV? Or is that something unrelated?