Home theater composite video (yellow) in? Xbox 360 5.1 audio?

I am really confused right now.

My 5.1 home theater system has RCA input (red,white) audio input, but it also has a yellow composite in next to each red,white input. I do not understand what this yellow one is for? I thought it is for video only?

I am trying to set up xbox 360 to have 5.1 audio. If I switch the xbox 360 cable to hdmi and plug in the three composite cables (yellow,white,red) to my receiver will I have 5.1?

My guess is that the yellow composite cable can be used to carry audio?
If the yellow composite in is not used for audio then why do I have them on my home theatre system which is meant for audio only? I assume there is a video out on it where you can plug it into a display?

Yes, a yellow jack on your receiver is for video only.

If you want 5.1 sound you normally need just ONE RCA cable from source to receiver. This carries a digital audio signal with Dolby Digital 5.1 encoding (also called AC-3), or sometimes DTS, Digital Theater Sound. Most commonly these use orange-colored RCA jacks, though sometimes they are black or other colors. However you can use that yellow RCA cable to make the connection.

More complexity: HDMI cables can carry the digital audio. If the HDMI cable is going from your Xbox to your receiver, and both the Xbox and the receiver can handle audio-over-HDMI, then you don't need any other cables to get your sound. This won't be the case if the HDMI goes straight to the TV, not if you want 5.1, because most TVs can only handle two channels.

Either way, the red and white jacks and cables are NOT used when you're running digital audio.