I started a Buffet mode world and created a world with only Lukewarm Water. I started in Creative mode and after a while I switched over to Survival, then the background music stopped. Turning back to Creative didn't get the music back. When day turned to night the music came back but when I switched gamemode again (from Survival to Creative, if I remember correctly) the music stopped once again. Re-starting the game (not just the world) turns the music on again.