music going quiet when turning on phone

I have my music playing when I’m doing something and when I turn my phone on to change the song or look at a notification or something, the music dims so much. And then slowly builds back up. And I really really hate it. But when I’m having a bad day and all I want is my music loud. When it dims feels the same as getting your pocket stuck in a door handle when you’re walking past it.

I don’t know how to turn it off and it really annoys me.