Even though I type in silence, I do immerse myself in my characters world as much as possible when I'm not writing, which means listening to their favorite music. In my last book, one of the characters was kind of emo, so I downloaded a whole bunch of obscure emo music and listened to it constantly. When I then sat down to write, I knew what songs were in my characters head and it was easier to channel the proper emotions of the scene.
The characters in my current book aren't emo, so I need a new soundtrack. Right now I'm listening to a blend of current pop songs that most teens listen to along with a few choice alternative artists. One of my more minor characters is in a indie band and these characters are definitely aware of the indie music scene. So I'm listening to indie music. It isn't always as polished as top 40 music, but somehow that makes it better. My characters are all a bit rough around the edges too, and listening to their music helps keep them that way.
What about you? Do you write to a soundtrack? And if you do put together WIP playlists, are your songs selected based on feel, lyrics, or character taste?