This has actually been going on since summer of 2010, which was the opening of the original WWHP in Orlando. It's not common, and many people have never noticed the various music arrangements slipping in and out of other audio loops.
Basically what happens is the major theme park giants of the area tap into their competitors audio, and play brief snippets of their park/attraction music as a way of subliminally attracting visitors from the competition onto their property.
"Hey Mom, Dad, I just heard Harry Potter music being played in front of GMR, (or on the bus back to the resort) can we go check out those rides next, please?" That's basically the goal.
This gimmick of hacking into audio systems and swapping music clips may have started with Harry Potter, but Disney has played the game as well to their competition. Once while at Universal Studios back in 2014, we clearly heard the Randy Newman score from Toy Story blasting momentarily while walking through the New York section near Mummy. It was brief, and very few people even noticed it occurring.
Sea World has also gotten in the game before. One time while at Epcot during F&W, we heard what sounded like the mating call of the beluga whale echoing just outside of the Germany pavilion. Subtle but clever, and apparently management at all major park chains has been unable to this day to stop the shtick or pinpoint those behind it... which is why it continues to occur.