The song "Happy Birthday to You" is an example of just how interesting the world of licensing is. Think about this song -- it is only 6 notes. Yet it is one of the best known songs in the world. It was written in 1893 by Mildred and Patty Hill and first published with the words, "Good morning to you".
The words "Happy Birthday to You" were first seen in print in 1924, although the author is unknown. Copyright was registered in 1934 in a court case involving a musical called "As Thousands Cheer" by Irving Berlin. The Clayton F. Summy Company became the song's publisher in 1935. Through a series of purchases and acquisitions, the song now belongs to AOL Time Warner. ASCAP represents the song for public performance licensing.
The copyright to "Happy Birthday to You" should have expired in 1991, but the Copyright Act of 1976 extended it, and the Copyright Term Extension Act of 1998 extended it again, so the song is protected until 2030 at least.
"Happy Birthday to You" brings in about $2 million per year in licensing fees according to this article. If you ever hear the song in a movie, TV show or commercial, a licensing fee has been paid. Any manufacturer making a toy that plays the song pays a licensing fee. The manufacturer of any musical card playing the song pays a licensing fee. And so on... This 6-note song is big business!