So assuming you have Xbox (video and audio) and CD player hooked into the Onkyo and then a lead going to the TV? In which case, three ways I can think of is:
(a) Re-assign the inputs and outputs on the Onkyo so the input you have for Xbox video is set to receive audio from the CD player;
(b) Unplug the Xbox from the Onkyo, hook it directly into the TV and set the TV input to the Xbox and Onkyo to the CD player;
(c) If you have an iPod, you can transfer the music you want to this, then hook it into the Xbox and play music from the iPod instead of the in-game music.
For the TV, easiest method would be to use TV's inbuilt tuner to watch the TV and set the Onkyo to the CD player / tuner input.