As a general rule, when writing I prefer silence. The wrong music can get into a writer's head and mess with mood, characterization, word flow, and all manner of things that shouldn't be messed with.
I do make exceptions, though. For my non-NaNoWriMo novel, I have playlists for all of my major characters, and sometimes playlists to describe their relationships. Not that I ever actually listen to those playlists while I write. Lou, however, put on the Rimsky-Korsakov Scheherazade one night while I worked on that novel, and now I can't hear Scheherazade without thinking of that story and seeing its images in my mind.
During November, I went silent except for the NaNoWriMo song. But here's my current writing music for this story:
Hayley Westenra's Celtic Treasure album
Owl City's Fireflies
ALL CAPS' rewrite of Fireflies as a Ron/Hermione duet
... with the last two sometimes alternating over and over, thanks to YouTube (and the linked videos are just wonderful, at least for geeks). At this point I'm probably obligated to buy both versions.
This means that Fireflies has run through my head for days, usually mixed up with the ALL CAPS version, so the lyrics will go something like "I'd get a thousand hugs/From ten thousand Viktor Krums" which is addled to the point of mildly disturbing. But I'm not complaining. I thoroughly love the song in both incarnations, and haven't tired of it yet. Besides, having it run through my head keeps all the annoying Christmas music out.
The original is also a good song for listening to when you can't sleep. I played it quite a few times between four and eight AM on Sunday.
As for Hayley Westenra, she could probably sing just about anything and it would suit my writing mood. But Celtic Treasure has Abide with Me on it, which I linked some time back. I love that piece. It still chokes me up sometimes.