Also, I've noticed the volume the music returns to after returning to the home screen is still lower than that if you were to quit the app. Test it out. Play a song, open PG, return home, then double tap home and quit. 3 entirely distinctive volumes.
I've railed Niantic about this. I feel it's their way possibly hindering people being too distracted and walking somewhere they shouldn't. Although loud music could potentially sever as a destruction, other forms of sounds are also decreased. Real life walking direction are hard to hear when a Pidgy is bumping around in a fictional ball.
I've read this in another forum: this is not a big, but instead a feature the developer turns on. Decreasing all other sounds decreases the function of multitasking. Multitasking is a core feature of any smart phone.
Anyone and Everyone should write in and complain to Apple and Niantic. Niantic needs to disable this in their code. Apple needs to make sure we have control over how apps alter our sounds.