AP STORY I hope this link works for a while.
While the story just says they hear notes in a special way that others don’t, we know that is code for “these people are mutants.” 
If this has already been posted I couldn’t find it and apologize.
I am a classical viola player in lots of professional orchestras. This article is absolutely right. I can remember not too many years ago when the “music” part still seemed out of reach because I was so concerned with where to put the fingers and other such technical things.
But there comes a point, if you work hard enough at it, at which you’re no longer playing an instrument. You’re playing music instead, and naturally hearing and understanding it in your mind’s ear. That is the sign of a budding professional.
Of course it’s the same way with any skill you learn. Driving a car, for example. It doesn’t take more than a couple months before your no longer “driving a car”, but “just going somewhere”. Make sense?..
And I’ll bet the brains of experienced car drivers (most of us) show that we’re not so preoccupied with the physical requirements of driving a car, but that we’re more aware of navigation, other traffic, etc.
It’s all part of mastering a skill.
All right, but how does that explain:
When musicians have a dinner party, how do you know which ones are the violists?
They have to be told when to come in.
What’s the difference between a seamstress and a flute player?
The seamstress tucks the frills…
I’m married to a professional musician. That article is no surprise at all. I just asked him if it was that way for him, and the answer is “yes.”
LOVE the musician jokes. Keep 'em coming.
Of course we’re wired differently. Some of you are wired for 110, but me, I want two 20’s. (preferably tall, well-muscled…sorry, I digress.)
I don’t know if I think differently, or if I hear music differently. I know I can’t go to sleep with music on, because I always have to listen to the whole song.
It’s also difficult for me to have music on during, uhm, intimate moments with my SO. I get distracted. Sad, but true. Not too romantic to find myself thinking, “Hmmm, I wonder if that’s a Roland or a Peavey they used for that keyboard sound…”
So how come some people have rhythm and some don’t? Why’s that, hmmmmm???
Life is so interesting, ain’t it?
We don’t keep the radio on during those (ahem) “intimate moments,” for the very same reason. When we’re riding in the car, my husband is perpetually talking about what gear they used to get that tone on that particular lick, blah blah blah yadda yadda. You know. 
Does this “hearing music in your head” thing include the phenomenon of not being able to get a song out of your head? I thought that happened to most people. I certainly don’t regard myself as having any musical skill or understanding, yet I can hear music in my head all the time. I need to consciously stop hearing music to get to sleep.