Video 1: https://youtu.be/Umry_VIw2eA
Video 2: https://youtu.be/OnYsRr4rnI8

I originally posted this issue as a comment on MC-36030, but although it could be related I now believe this to be a separate issue.

There is an issue with the sound engine in the 1.9 prereleases (tested on 1.9-pre3 and 1.9-pre4) where sound channels (in this case, music channels) will not get unloaded in specific circumstances. In the first video I encountered it quite by accident, but in the second video I figured out how to make it happen with music channels. Here are the steps to reproduce it using music:

1) Open any Minecraft world and wait for a song to begin playing
2) Wait 20 seconds (ish) before exiting the world
3) Menu music should immediately begin to play
4) Go back into the same world and you will notice that the music will immediately start again, stop, then start over
5) This music channel is now bugged. You can pause the game or exit the world and it will continue to play. Additionally, the music slider will not work on it

Also in my first video, I somehow managed to get the guardian laser sound to behave in this manner by using a /kill command near a guardian temple. That proves that this issue does not specifically affect music.

I have linked the log produced by messing around in the second video, which I set to development mode (so it logged changes in the sound channels).