ok, you need a combonation of software and equipment to do this. SoundJam MP (or whatever I'm thinking of that does this) is Software, but it needs your song to be on your hard drive first (in aiff format, or whatever). You'll have to capture your sound in one way or another, then encode that into an mp3 using SoundJam (or whatever mp3 encoder you may use).

This is why I'm putting about $3000 into my current computer as soon as I get my PowerMac G(5, 6, 7, who knows (considering the rate at which I earn money)).