I was 54 yrs old before I discovered that other people DID NOT hear music all the time - and I mean, all the time! I thought it was normal and so never really understood the appeal of CD players strapped to your arm or iPods in your pocket. Unless I am concentrating, such as reading a book or standing over a stove cooking, I always have music running through my mind.
Sometimes it's the mention of something that sets it off, but not always. I will just wake up in the morning and some stupid chorus will start endlessly repeating...for days on end. What drives me crazy (figuratively, of course) is that it's often songs I don't particularly like.
It's called "musical hallucinosis" or "musical hallucination". More women are affected than men. It's harmless, but distracting (since I don't have epilepsy, brain lesions, or alcoholism, which are the most common factors). It's why I loved doing Tai Chi, but can't sit and meditate, LOL.