I am not sure of "behind the scenes" when it comes to music in games, but I know if you want certain music then it usually means paying a fee. I know there is royalty free music, but I think it still requires a nod in some way to the ones who made it. I do know having music in the game adds atmosphere, but it brings up issues: the song didn't fit here, I don't like this genre, that band sucks, I want this band, etc. I don't know the decision Nunu has for music and CBTL, but I can tell you it is a genuine reason.