You have to pay for [mp3].
Mp3 does incur royalties, but in terms of the Blitz programmer just getting a reasonable-selling shareware title out, it's not so bad:
http://mp3licensing.com/royalty/games.html
No license fees are due if less than 5 000 copies of a particular game title are distributed.
Not that I agree with file formats or algorithms being 'protected' in this way, but by the usual Corporate Evil standards, that's pretty decent, and how many Blitzers have sold more than 5000 copies of a single game? For over 5000 copies you're talking $0.50 per copy.
(At that point, you just switch to .ogg, which you should have done from the start.)