For me it's a mix of Classical and American Jazz from before around 1950. I don't know why. But sometimes when I'm working a scene setting up my view camera, looking at the scene upside down and backwards on the ground glass (but seeing it right side up) and working the controls by feel, I do hear music. Everything from J. S. Bach to Duke Ellington. A fair amount of Mozart and Haydn. I seem to have a gap between Beethoven in classical and Fletcher Henderson in jazz (about a 40-50 year gap). Again, don't know why.
But I do know that when I go revisit a scene to make another photograph, I hear the same music. It's like they are associated somehow in my brain. Makes me think there might be an electro-chemical reason for hearing music while I'm lost in photography.
I was interested enough in this that I actually exchanged some letters with Oliver Sacks (neurologist and author of a number of books including Musicophilia which I highly recommend to anyone interested in how humans deal with music). He told me that it's not unusual but is somewhat rare, and that he doesn't think it's a form of synesthesia. Which sort of disappointed me.