I'm one of those people who actually concentrates better with music. It allows me to ignore ambient sounds, and focus entirely on what is in front of me. The effect is amplified when I have ear buds or headphones on, because I exist in my own sound bubble. I've had some people be confused this, because they get distracted by the music. I groove with it.
Now, the most obvious explanation is to use music to emphasize the tone of the piece I'm working on. If I'm working on something serious, perhaps Mozart's Jupiter Symphony might be appropriate. If I'm working on something melancholic, some Fleet Foxes, or if something happy and cheerful is needed, Katrina and the Waves "Walking on Sunshine".
I am an oddity. I don't listen to music that is necessarily the same tone of what I'm working on. Sure, sometimes I will align my music with my writing, but most of the time, if the music gets me into a groove, it can be the exact opposite in tone and still work.
I wrote several academic papers last December, listening to German Christmas Carols and Auld Lang Syne ad nauseum (to the point where you might believe that I should know the lyrics to the latter. I don't). If I tried to listen to something else, the farthest I was able to venture was other traditional Christmas Carols. I'm not even particularly fond of Christmas Carols, but they were the only thing that kept me focused enough to slog through my papers. I wrote a paper last month listening to one single song: Bastille ft. Ella -No Angels. Was it at all relevant to my topic? Not in the least.
Working on The Brightest Night, I've listened to club music, boogie/funk/disco, atmospheric, folk, rock, pop, and everything and anything in between. While I still get distracted as much as the next person, it's not the extent that I would be if I didn't have my jams playing.
It leads me to wonder, if music helps set the tone in film, what role does it play during the writing process?