My nephew forwarded a link to a discussion about music to write by. I also write to music and clearly this is a matter of taste--what gets your creative juices flowing? Especially with fiction, music must not intrude upon the creative flow. For me, that means no voice, only instrumental music. Well, let me clarify--no voice singing lyrics in English that I can understand. So someone singing in a language other than English might be okay.
It also depends on what kind of piece I'm writing. When I'm blogging, like right now, I can listen to lyrics which are very repetitive. For example, I'm listening to Moby 18 as I write this. I like Moby because each song has such a definitive mood.
Mood is extremely important to music to write by. Perhaps that's because music can inspire a deeper reflection on the psychology of character. This is often true as I listen to Philip Glass while writing--my very favorite composer to write by. I also like Bruckner. Wagner and Mahler are too much. Vivaldi is great, but Mozart is too much. Modern musicians like Brian Eno and Raphael are wonderful, too.