In game music??? Something new for a game possibly!
Typically in these type of games there isn't any sort of music unless you're running by something that is broadcasting noise or it's time to fight!
But since this is (basically) modern day this idea is possible. Why not add some sort of "iPod" feature where, if you chose to, you can hook up your headphones and jam. I know I would like to get a song going before raiding a building full of enemies! Though risk do include not being able to hear people coming up on you. Maybe a volume adjustment feature?
You could even make it like some sort of scavenge type system... i.e. Finding a thumb drive throughout the city while you're already going through everything already. But here's the catch, you can't see what is on the drive until you go back to HQ to plug into a computer there. Sometimes you get a song(s) and sometimes you get nothing. Random footage of life in the DZ (if found there) to get more background story understanding would even be cool.
There are many games that could do this and increase player enjoyment while running around exploring. Why not give it a shot?
Food for thought, can't wait for the game! :D