I'd say it's more from the pushing on the sides of my head, not so much from the volume of the music. But you are right, and I have been thinking about stopping listening to music trough headphones when working, and use speakers.
> If you're sitting with headphones on for six hours, maybe you should get up a few times.
I have to say that my current working desk and rhythm is pretty much perfect as far as I know. My eyes never get tired, the screens are a good distance away from me, and I don't have to look down. I have a decent computer chair, and I take breaks about every hour, to go outside or get a new cup of coffee (but I do that with my headphones/earbuds in).
Heck, no employee can offer a better working situation if you ask me, I'm all for the working remote - office not required-idea! :P