There are a few apps to add this and I found no sign it required rooting.
I have a Galaxy S9 and often have music playing on Spotify while using Pokemon Go.
Unfortunately, turning up Spotify music volume also turns up the very obnoxious Pokemon Go sounds, which I want to STAY ON so I can HEAR if anything pops up if I'm not looking at the screen.
I know there are apps that set the default volume of apps when they're opened or closed, but are there any that can, for example, allow Spotify to be on max volume while Pokemon Go is at like 30% volume, with them running at the SAME time?
Also needs to be something that does NOT require rooting the phone.