Inspired by my wife, my muse.
There are several medical reasons to have music in your head. One is called “musical ear syndrome,” which is a type of hallucination most commonly associated with hearing loss. Another is associated with psychiatric disease, especially obsessive-compulsive disorder, but also with schizophrenia or mood disorders.
However, my experience is that there are many, many normal people with no psychiatric illness who have music playing in their head almost all the time or all the time. Many people have had a song stuck in their head (often called an “earworm”), but a few people have this continuously.