Ok, I've been doing some researching...
Check that site out. It's an open source program called Audacity. It's super sick for recording sounds and music.
If you can record multiple inputs on your computer, you can just play the mp3's and talk over them. The program can then encode it to mp3 and you're set!
If not, then it's easy to lay down your music, and then play it back and talk overtop of it!
Give it a try, and I'll keep looking for more programs too.
I'll also put together a tutorial on how to do it!
It's only cool after it's played out.