|I think this may be physically impossible.|
If writing is my favorite thing in the world (debatable anyway), then music is the "other thing". Ironically, used to be that I couldn't write to any sort of noise in the background, music included. Slowly I worked my way up to ~new age~ white noise/elevator music, and now I can slam out a love scene set to Dethklok if that's what it's all come down to. (I wonder how I would write such a scene...perhaps I will try one day?)
I do keep my background music pretty separated when I am writing, however. If I'm working on Nagnomei, odds are I'm listening to Euro-metal. If I am working on CROSS//, I am probably listening to anything that's not Euro-metal. Atmospheres and moods and all that snazzy stuff.
Theme wise, I have picked out songs that I could see being the themes for each book I've written, and I do put together make-shift soundtrack for them. The really odd thing here, though, is that I rarely listen to them while I'm writing. It's more like what songs I imagine being ina movie/series adaptation of each novel. There are certain songs that have inspired certain scenes that I will put onto repeat while I am writing them, but otherwise, I'm not quite that limited. In instances of writing, music is only background white noise for me to be amused with.
For fun, here are the songs I consider to be the themes for the two drafts I am working on now:
Nagnomei: The Key of Nixey