you can get a RCA to mini-audio connector at any electronics store like best buy, circuit city, radio shack. it lets you plug thr red and white RCA cable from yout TV sound input to you sound card's mini-audio output which is typically color coded green.
its the same output that you use to connect PC speakers or headphone to. make sure you have the sound input on the same set as where you are connectiing the video output from video card. you are basically setting this up as if your computer were a home VCR/ DVD player with exception of the mini connector.