That’s your mind’s ear! It can be more vivid or less vivid depending on the person. Neat trick I learned in college that everyone in the room could do (we were all musicians, so may be biased). You can take any melody and hear it as a different instrument. You can make yourself hear it as a flute or as a guitar or as a trumpet. And getting good at this skill can make it so you can hear entire fully harmonized orchestras in your head on command. It’s wild.

Edit: I suppose the specific instruments you can hear would be culturally dependent just because of frequency of exposure. Like I’m pretty good at hearing a full metal band playing including what effects are on what instruments, but I have a hard time imagining a duduk play a simple melody.