OK here I made the Diagram. You might also wanna look on JaredC01's diagram
to get a better idea.
I interpreted JaredC01 diagram to explain how to do this...look
This one is for just plain audio straight from the Xbox... un-changed, un-amplified. So using this setup is easy, but the sound is not loud enough in most games.
This one is basically the same but with a amplifier. Notice, that adding a amplifier not only the sound will be louder, but you could add a volume control to it.
If you plan on installing a amplifier you can draw power from the Xbox PSU to power it up.
Also, I noticed that when you add the headphone jack and connect the headphones to it, the Xbox will cancel out the audio from the speakers.
Hope this helps.
***Note: I am not taking credit for JaredC01 diagram. I just made one myself to explain how to connect headphone using the exact same pin numbering on Jared's diagram.***