This bug has been in the game since 1.7, the in game music doesn't stop when a jukebox starts playing, allowing music mixing. This also happens when one jukebox is playing, and someone puts a disc in another one. The music mixes!

Here is the video of the bug in action: https://www.youtube.com/watch?v=uyJ6Qg6Wwc8

(I uploaded it to youtube because the file was too large)