Outputting tv audio through pc

[saads]

Hi,
I have my PC connected to a set of logitech z-560 speakers. I want to output all the sound from my TV through my PC to get the surround sound from the logitech speakers. I connected the audio outs of my TV to my soundblaster live line-in (using an RCA to stereo 1/8" converter), but I only get sound from the front two speakers, no sound from the rears. Surround sound is fine when playing media from the PC. Any ideas on how to get around this? The sound blaster live is an old card... over 3 years old. Anyone know if this would be solved by getting a new card?
thanks.

 

[saads]

I guess more concretely the question is: why is the soundcard able to decode pc media (mp3s, divx, etc) into 4.1 channel surround whereas it can't do the same to the line-in stereo input? I have my soundblaster speaker configuration set-up to use the 4 speakers and so I don't understand why it doesn't decode the line-in stereo into 4-channel surround.