Changing your gamemode to creative from survival by using the command /gamemode 1 abruptly cuts off any background music that may have been playing.
Steps to reproduce:
1. Go into survival mode.
2. Use the playsound command to play one of the music tracks, or wait until background music plays.
3. Use the command /gamemode 1
4. Observe that the music abruptly stops.
Sorry. I searched for a duplicate of this issue before creating it, but I must have used the wrong keywords, instead of the ones for the duplicate of this. I know that over 50% of issues are resolved as duplicates, but I wasn't able to find it, as I said.