2709w, TB Santa Cruz, xbox360 setup question

[HFrook]

I just got the 2709w Panel and would like to output the sound to my computer speakers. Right now I have a cable going from the Audio out on the panel to the line in on my TB Santa Cruz card but sound is only coming through the left speaker. The speakers are just 2.1 Altec lansings. I have the 360 hooked to the panel via hdmi and was told that the sound would just pass through so i could hook a cable from the audio out on the panel to my sound card and it would work but its not. Please help.