You put a bunch of MIDI filenames in a file called playlist in your Squeak directory, then tell [MIDIJukeBox startPlaying] fork. When you want it to stop, do MIDIJukeBox stopPlaying.
Betaware as of 12/2/98, but I'm trying it, so maybe I'll find the bugs first :-)
We're using it for our Cooperative Radio (see http://guzdial.cc.gatech.edu:8080/squeakers.47), so this version expects to find a page named 'playlist' in a Swiki named 'Jukebox'. You'll need to tweak this for other uses. Mark Guzdial