The "Happy Birthday" song is actually still protected by copyright which is why so many restaurant chains come up with really annoying alternate versions: no royalties.
I'm not sure when the copyright expires, but at the rate our elected officials are voting for the highest dollar, it may get extended yet again.
In any case, there's a
page at snopes.com that covers the song's history and current status.
That's not to say that you won't be able to find a MIDI file of it somewhere - it is the most sung song in the world, after all, and royalty fees are normally collected from large commercial entities such as films and restaurants.
