It’s still synesthesia. It’s really, really cool, and I wish more people could experience it (I wrote about it earlier here https://news.ycombinator.com/item?id=15440878).

That sounds closer! It's almost like an involuntary visualization. It's just how I've always heard music though, not related to anything psychoactive.

I remember seeing a discovery channel documentary where they put a musician under an MRI and a lot of his brain lit up.

So I suspect it’s actually part of the common human experience and not synesthesia.

If you would actually see it, IRL then that would be synesthesia.

If you see it in your mind’s eye as vividly as you see things IRL then it depends.

Disclaimer: not an expert, I simply have a small interest in these topics. During my bachelor synesthesia was perhaps covered for 5 minutes. I learned more about it on my own.

I was inclined to believe it's normal, but you never know unless you ask!

I remember seeing the guy who can calculate a bunch of digits of pi on the discovery channel as well. He also had synesthesia. I always wondered if there was a trick to what he was doing...

How do people distinguish between “actually seeing it” and a visualization?

I’ve heard it being used to memorize the digits of pi, so it’s clearly extraordinary, but it’s difficult to see what is a normal reaction to music in the first place. I can’t imagine “just” perceiving the music at all.

Lucid dreams are much more akin to seeing it than whatever I “see” in my minds eye.

It’s not like the Hololens but I can still tell you the shape, color, distance, location, texture. Since I can basically switch it on and off, I can tell you that the normal experience of music doesn’t come close (and I always loved music).