Are you sure? I haven't seen many TV's these connections. If you do, all you need is a stereo lead
Skandia
And a splitter:
Audio Splitter - $8.99 : Safenet Computer!, The Art of Computer
You have to be careful, there are two types of audio splitter, one which splits the left channel from the right, and others, like the one you need which splits both.
You can buy these from most electrical stores, Curries, Dixons e.t.c.
Plug the splitter into your sound card where your speakers are now. Plug your speakers and the audio cable into the splitter and away you go!