The current music system forces people to listen to the battle music, unless they turn off music in the music options. This however, disables any sounds played under ''music''. My suggestion is to add an option in the hocon file for both trainers and wild Pokémon separately.
Another idea is to, like the skin of the npc trainer, is to be able to enter the name of the sound you want played when you interact / during battle. For example, if you want "music.versus.rival" played you enter that in the bar in the NPC editor GUI. Another option is to just be able to turn it off / on depending on the trainer in the NPC editor GUI.
The reason for me suggesting this is that I'm making a custom map, and I've set music using Trainer Commands directing to command blocks. The music is a good idea and would definitely be improved a lot with these features. The way it is now is really limiting.