When I heard the new music had been released, I immediately went to my computer and started up the game. I got to the title page and no music started playing. I proceeded to select one of my maps and begin playing. The new music still didn't start playing. I switched between versions and profiles via the launcher and I still didn't get any of the new music. I proceeded into the Nether as well as The End and still nothing. At one point while I was playing, now going about my own business not worrying about there being no new music, two of the current music tracks started playing over each other. I've since had my music turned off. I don't believe there's a way to trigger it, it just happens.