I'm curious about the 990 to TV connection. The 990's component monitor output (the green-blue-red RCA jacks labeled "MON OUT" and located next to the RS-232 port) should be connected to the TV's component input. The component output and input are video only - they carry no audio. If you want audio to also go to the TV, you would probably want to either use one of the digital audio outputs (assuming your TV has a digital input) or use an audio output such as the AUX or TAPE record outputs (located beneath the OPR/SVC switch, which is directly below the MON OUT).

I'm also curious why you are sending audio to your TV - with the digital audio from the XBox, I'd think it would be preferable to let the 990 (and whatever amp and speakers are fed from it) handle the audio.