Aphantasic here - I constantly have music playing in my head to the point of it being slightly annoying. If I really concentrate, I can separate out the parts like stem separation, but generally it's a fairly high fidelity recreation of the most prominent parts. There was a brilliant thread on Mastodon a couple of years ago where people (mostly musicians) were describing their inner-music abilities - mine turned out to be very ordinary. The only time mine ever approach what many of the others described is when I'm very slightly stoned - then my brain starts to compose original music at extremely high fidelity in basically any genre. I just wish I could get it out of there.