As MCE2K5 says - that looks like a TV tuner card - is it digital or analogue?
Yellow is picture - red and white are sound. If you are also using s.video for the picture, you may va an input selection issue on the TV.
Without seeing all the documentation it's difficult to offer advice.
Here's one simple test you may be able to do. If you have some computer speakers or headphones, plug the sound output into them, and see if you get sound. (you may need an adapter to get the correct connections.
If you get sound then - the computer end is OK. If you don't, then clearly the problem is at the TV card end.