By use, I mean when you set your music before battling in the Ultra Sun and Moon Battle Tree.
The special NPCs I am referring to are Wally, Cynthia, Grimly, and Colress. I don't care about the others :P
I know you battle a random one in Super Singles at battle number 10, 20, 30, 40, and 50; however, when I had "Necrozma Version 2" Selected as my battle song, and fought Wally/Colress in my [Recently ended :(] 35 win streak, they played Necrozma Version 2 instead of their theme and I didn't unlock either of their theme in the "Battle Music" menu. Do I have to use the default music against them? Or is there some weird thing where I fight them elsewhere? :P