Do you have another audio cable to try out? Often those things tend to short out and one or both channels will die because of the loose connection.
Now, the other day I sent my brother to get a 3.5mm cable because the one in our car had died and was only playing 1 channel. he came back with another and we had the same problem. We concluded that the jack in the car was broken and were displeased.
It wasn't until 2 days later when I was throwing away the box that I noticed that it was a mono cable. My brother was given a hard time.
If the end of your cable has 3 metal sections then it's stereo, if it only has 2 then it's mono. What say you?
@AndrewB: No I've been using the one I currently have since I first started connecting my TV to my PC, you think its worth just biting the bullet and getting a new one?
An RCA 6ft stereo cable is a bit north of $3 shipped for me from Monoprice (the actual cable is 85 cents). If you have a local shop that's cheaper, even better. Couldn't hurt in your quest to narrow down the cause.