It doesn't help that my local coffeehouse has TERRIBLE music. (A fact that I've gone on record about multiple times)
This of course, leads me to use my iPhone and a pair of earbuds to tune out the rest of the world as I type frantically in my usual 'hunt and peck' meets 'ingrained pattern' on the keyboard style. I doubt very much that my 7th grade typing teacher would be very happy with me if she saw the haphazard way I've boosted myself to my meager 50 wpm. She certainly wouldn't have approved of me listening to music and bopping my head while I do so.
The choice of music for me was a very personal one. I tend to listen to upbeat tempo songs while writing, mixed with a bunch of Billy Joel, Bruce Springsteen, and Paul Simon. I know other writers that change it up depending on the scene they're writing, and while I can see how moody music would effect the tone of what you're writing, I have also found that it doesn't really work for me. One friend in particular had created seperate Pandora stations for each character in his NaNoWriMo novel, and would alternate between the stations as he wrote their scenes.
I guess I'm lucky that I've chosen a tone for most of my work that reflects my style, and that style I think is reflected in the music I write to. I could easily see myself getting obsessed with picking out the perfect song/songs for each scene otherwise, spending all my time trying to capture the setting in someone else's music instead of my own mind. I doubt I would get much writing done that way.
Having said that, I do pick out upbeat songs for the simple reason that I find myself trying to type to the beat, which always helps me type faster, and get more ideas down on the page ion less time that I normally would. I've also "hacked my brain" into prepping my imagination when I hear a particular song.
I discovered this effect quite by accident. with my old iPod, i had no control over things like "shuffle" or "repeat", primarily because I refused to read the manual Cheri gave me with her hand-me down iPod. "Come on honey!" I proudly declared, "Where would man be without the adventure of discovery! Would Columbus have really been better off with a Garmin on the dash of the Santa Maria telling him how to find the west indies?"
At this point my wife launched into a tirade about the attrocities of Columbus, which I will save for another time.
Anyway, with no way of controlling my song order, I had to listen from song 1 to song 2, etc through to the end, where I would restart the ipod if I wasn't done, just like my good old walkman cassette player in high school. This was oddly comforting to me as I would start writing. I later figured out shuffle, but my playlist would always start on the first song. I noticed that whenever I listened to that first song, my brain would start to get into writer's mode, just like Pavlov's dogs had he been sitting them at typewriters instead of feeding them.
Of course, one would imagine Pavlov's mutts ripping him to shreds after weeks of getting typewriters instead of kibble...
In any event, the song at the top of my playlist is currently (and appropriately, I think) "Why don't you get a job" By the Offspring: