Venom2009

Hi guys, been browsing the forums for years, have enjoyed the posts and learned much. I have actually ran into a problem i cannot figure out for the life of me, and was hoping someone may have ran into this before.

What I am trying to accomplish is running 5.1 surround sound on a ps3 through my computer. The items i am using are as follows.
Computer with Xfi Music edition
Ps3 hooked up through HDMI to a Toshiba 40 inch Regza TV
z560's (may be 480's pretty old, but they are 4.1 speakers)

My idea was to send the sound source from the tv (red / white audio out ports) to the line in port on the Xfi and setup the center speaker to use the TV the front and rear speakers to use the z560(480's whichever they are lol).

Is this possible? or am i wasting my time. I can get it somewhat to work, but there's a horrible echo coming from the tv(maybe it's trying to run the hdmi audio and audio from computer at same time?)

Thank you for your time.
D