Is it possible play mp3 files from a computer thru a stereo? If so how would one go about wiring this?

How about connecting the line-out of your soundcard to the line-in of your stereo ?

If your stereo is more than a few feet from your computer you will have to buy an external soundcard. I tried connecting mine in the next room with 25+ foot of cable and what little I managed to hear sounded like shit.

I got an external soundcard and 15 feet of USB cable and now it sounds fine. There are several out there, do some searching. I got an Edirol UA3, there's also something called Stereolink and some others.

I've used a 20 foot long cable by Monster Cable and my results sounded like music.