the problem is this.

Encrypt the MP3. o.k thats possible.

only you know the key. but this means only you can listne to it. so this is impractical if you are selling it.

you could make it so that only a certain player will play it, but you'd have to write the code for this into the MP3 file.

if you give the mp3 a key that you then supply the user with, the moment they use the key they can copy it and its game over.

i gnarly problem indeed.