First, the Summer seasonal OST that plays in Metropolis when you first enter. Now, the Winter Seasonal music that plays in Metropolis, and also the one from Gotham too. It's not the same going around Gotham/Metropolis without that festive track playing at this time of year. (This excludes the event boss theme and the OST when you encounter adds during the open world mission)
If I read the Hotfix notes right, the decorations (and I assume that means the music also) will be turned on later this month. It has something to do with how they're running it this year. Larfleeze stealing the decorations or something like that.
