Are you running the line out of your amp.. Into the microphone jack on your PC?
If so, perhaps it's an impedance problem?
Plug a set of headphones into the jack that you're running TO the PC, and see if you get sound. If not - that's the problem - try using the headphone jack (if there is one) instead. Otherwise you'll need either some sort of converter, or better yet a "real" soundcard IN for your PC.
M-Audio makes tons of great stuff. Some swear by the Audiophile line, while I use the preouts on my GSP2101 to an M-Audio Delta-44 breakout box.
Give me a little more info on your actual setup and I'll see if I can figure it out.