you'll need to get some sort of audio to usb adapter, like the griffin iMic( <a href="http://www.griffintechnology.com/products/imic/index.html" target="_blank">http://www.griffintechnology.com/products/imic/index.html</a> ). the only firewire solutions i know of are MOTU's 828, but that will run you about $700.
griffin also used to have a product called powerwave, but looking at their new site, i think they discontinued it. here's a link to the old product page: <a href="http://www.griffintechnology.com/audio/pwrwave.html" target="_blank">http://www.griffintechnology.com/audio/pwrwave.html</a>
a little pricey for what you want to do...your best bet is the iMic and a stereo RCA-1/8" stereo male cable.
one question: are your tv speakers really that bad? and i assume your tv and iMac are close to each other, right? because watching a tv in front of you, while the sound comes from your iMac across the room would be kinda annoying!
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