All you need to do is buy the advanced s-video pack for xbox. It has s-video out, composite(yellow) video out, left/right audio, and Digital 5.1 optical output. All you need after that is the optical audio cable.
I don't know your understanding of xbox/products, so here's some more info:
The advanced pack is just a different video connector sold by MS almost anywhere an xbox is sold.
There is also an HDTV pack(Hi-Def) with a 5.1 optial audio output.
Monster Cable also has their own line of "Monster Game" cables for xbox, but have unusual 5.1 outputs requiring a second purchase to complete the product.
If you don't have an HDTV, then get the S-video pack as it has 2 common types of video outputs and the 5.1 digital out you're looking for.
Both packs run around $19 US while you can find 3rd party brands for less, but you get what you pay for. The Monster Cable stuff is closer to $50 for HDTV and I think around $30 for S-video.