When you say "stream audio," what are you streaming to?
If you're trying to stream music across your home network to other computers, PC's with Windows 7 will be able to see each other in Windows Media Player and play music stored on other PC's. If you have PC's with other versions of Windows on them, a simple folder share across the network will suffice.
If you're attempting to connect your computer directly to a stereo receiver, depending on the inputs on your receiver and the outputs on your PC, the best choice would be an RCA to headphone cable. The headphone jack plugs into your computer, and the RCA (the red and white/black plugs) go into your stereo. These cables are dirt cheap, you can find them in any RadioShack or BestBuy, but online will save you a dollar or two.
If you're trying to stream music to your PC from your receiver, that's a long explanation I'd rather not go through unless necessary.
PudgyChicken