Pretty much most of my life I thought everyone had this (as well as vivid imagination) but now I dont think its as common. Like I can cue up any song in my head and it plays through, all the instruments relatively separated etc. A similar thing happens for writing music, I can write song after song and rarely feel like I “dont have any ideas”.
Sometimes I wish that power could actually be utilized…
I have a friend who notices when she forgets her adhd meds because the music starts playing.
When she told me this it was the first time I realized constant brain music is not a normal thing that everyone has.