These might be a dumb & basic questions but:

1. Does your X box display over the S video connection properly (but not HD)?

2. Have you configured the video input (Video1, Video2 etc) that your X box is using in the setup menu of the 990 to be COMP1 ? If you have not, I can walk you through this.

3. Are your 990 monitor out cables to the TV really component cables? RCA and component video cables are NOT the same thing, just the jacks. Make sure you have 3 lead component video cables and not just a three lead RCA cable where one is COMPOSITE video and the others are stereo RCA.

I am not quite sure why you are running an audio feed to the TV. Maybe you want the option of having X box sound from the TV speakers, but this is another issue and sound not affect the video.