Try to study the “Caribbean Trade War” or “Dragon War” maps I made. They have specific player music. Every nation has 1 music file that is played every time it is that players turn.
All the sound files are located in the “sounds” folder, but the playing of them is controlled by editing the “sounds.properties” file.
PS: I would keep the length of music down to 30-120 seconds. That is the length I aim for, and I am hoping that this ensures that a player will most likely be done with the specific nation’s actions before maybe having to handle a new nation or the same nation again. There is a risk of sounds/music being played on top of each other. And this does not sound very nice.
PPS: I have tried to always list instructions in the map notes on how to turn off the music.
PPPS: I have tried to also have the volume of the music a bit lower than the other sounds. I think the music might irritate some players and motivate them to just turn of the speakers.