If you put a music disc into a jukebox before exiting a world, the music will no longer be playing when the world is re-entered. However, the jukebox continues to emit the note particles as if its still playing the disc. It continues playing until the point when the song should have ended. In the photo with two jukeboxes, I put a disc into the right jukebox before the disc in the left jukebox ended, then exited my world. This shows the notes continue as the disc plays but stop when the song ends.