I don’t really understand why the question is so common, but writers are repeatedly asked what (if any) music they listen to while writing. It comes up in the #NaNoWriMo Twitter conversations. It comes up in blog posts (like this one yesterday from Austin local and WordPress/NaNo stalwart Jackie Dana), in author interviews, and almost any occasion a writer fields questions.
Generally, I’m in the “sometimes listen to music before writing, but rarely while writing” camp. I spent most of my adult life studying music, and even the wallpaper Baroque and early classical performances that so many people use as writing or study music can distract me. (Or just annoy me. There’s a reason I specialized in 20th-century avant-garde music.) There are exceptions, though. When working on a scene from Ghosts of the Old City that I’ve since discarded, I put on György Ligeti’s Atmospheres. It’s dissonant atonal stuff, heavy on strings and built around texture. It was the right music for the scene in question.
And that’s my cue to pivot. I’ve spent the first part of Camp NaNoWriMo cleaning up my outline. (I’ve historically been a pantser, but the middle chunk of Ghosts was a horrible soup until I set aside the time to give it structure.) I was surprised to find myself mentally scoring the scenes to help pin down their content. My thoughts about the scenes’ moods were more musical than verbal.
I started grad school as a composer. I started as a composer because I dug music. And largely, I dug non-pop music because of film scores. I spent a lot of my undergraduate years composing pieces with explicit or implicit narratives. I also spent a lot of time thinking about music and text and how they worked differently for telling stories.
The upshot of this is that my ideas of textual structure are thoroughly tangled with my ideas of musical structure. When I think about pacing, I imagine a conductor’s gestures. I think about conflict in terms of crescendoes and cadences and shifts in orchestration. Back when I composed long pieces, I’d get one or two pieces of 11″x17″ paper and sketch out the shape of the piece. Sometimes I’d do that with a carefully-measured timeline across the top of the pages, others I’d just be roughing it out—“trumpet solo here,” “percussion cacophony,” “pianissimo strings…”. I’d draw shapes and small pictures on the page. I don’t do that with my stories, but my longhand outlines sometimes get close as visualization exercises.
Music plays out in time. In a live performance, at least, you can’t flip back to check if the theme you’re hearing now is related to the one you heard two minutes in. (You can do it with recordings, but unless you’re a music student, I think it’s pretty rare.) It’s the composer’s and performer’s job to make those connections clear, to hold the listener’s attention. You get one shot at structure, start to finish. You can use inherited forms, many of which are based on returning material and/or harmonic progressions, or you can make your own structure. Either way, the structure is experienced linearly.
Books don’t have that limitation. You can read them by flipping around. You can read the end first. You can go back and remind yourself what happened in the first chapter. Despite that difference in the medium, structure in fiction works an awful lot like musical structure. Even if you break up your plot, reveal it in out of sequence bits and pieces, it has to be compelling as it unfolds for the reader. You have to bring them with you through the story. The crescendoes have to get them excited. If you change keys for the bridge, you’d better have a good reason (and probably ought to consider bringing some of the bridge material back to prove its relevance).
A novel isn’t a symphony. The concepts and themes to balance are different. The techniques are different. For me, though, it seems more and more like the principles are the same. Whether you’re writing a story or a string quartet, you’re giving shape to chaos. You’re making the inchoate intelligible. Words or notes, you’re paving a path for your audience.
Long story short, there’s a lot of music involved with my writing even when my speakers aren’t making a sound.
Walking Ledges 
In college, I always had music on in the background … usually classical. Think part of it was purely white noise, although I did learn a lot of compositions through the repeated playings.
In the newspaper office, I finally started using earbuds and streaming to blank out the TV overhead. Opera or Q2 (living composers) was far more conducive to concentration than the hysteria that passes for a newscast these days.
At home though, I usually find music distracting while writing.
Your comments on symphonic structure and sonata form, though, ring true for me.