I suppose it is a viable option (if I did my math right, about 700kb).

Only thing I'm confused about is how I would set this all up. For one thing, JEDI-SDL seems to be using an older version of SDL, is it compatible enough so I can just download the latest SDL_Mixer libs from the website (1.2.11), include sdl and sdl_mixer, and expect everything to work? Or am I going to have to find an older version of sdl_mixer to match?

Edit:
Rather than using Jedi-SDL, I used the SDL packaged with FPC (which I imagine may be at least part jedi-sdl). Copied over the latest libs, and after an hour or so of tinkering I've got it playing MP3's happily! Thanks for the idea Stoney. (Although currently the total extra size seems to be 1.1mb, we'll see if I can't trim some fat)