Music for deepdark biome doesnt play correctly on creative mode, the easiest way to prove this is staying in deep dark for a while, the music for this biome will never play.
When you load a world the music will play after some seconds so is easy to reproduce.
In the video I recorded I stayed afk in the deepdark until the music played.
I wasn't able to reproduce, can you please provide how long you have to stay in the deep dark until music/sound track stops playing? Relates to MC-272032 which was resolved as cannot reproduce.