Same issue on my Nexus 6P.

I'm not an Android developer and could be way off here, but since there are separate volume controls for notifications and music, it seems apps have a choice to play a sound as a notification or music. In most apps, small noises are played as notifications and thus the notifications volume controls them, and notifications from any app may cause background music to quiet or pause depending on the apps' audio ducking settings. But two apps can't be playing music at the same time, it seems. I can't play music from Spotify and Audible at the same time. Same it seems for Pokemon GO and any music, as the Pokemon GO sounds are all music. If Pokemon GO sounds were notifications, they could go over the top of music or an audiobook played from another app. This is why starting Pokemon GO pauses any music playing from other apps, and why starting that music again will stop the sound from Pokemon GO.

I think if Pokemon GO left the background music implemented as music but implemented all the other sounds as notifications, we would be able to play music or listen to an audiobook while still getting the Pokemon GO sound effects, just not the background music, which I have turned off in the settings anyway. If they made that change, you would then not be able to quiet all other app notifications and only hear Pokemon GO effects, as the notifications volume control would then control everything except perhaps background music from the game, but I think that would be fine.

I definitely prefer to be able to listen to audiobooks while hearing Pokemon GO sound effects, even if I also hear other sounds from other apps. The one problem with that is sometimes such notification sounds still screw up my audiobook, but I think that's glitches with Android itself, as I've always had that problem. It seems if I get two notifications too close together, something goes wrong with the command to resume the previously playing music and it will get stuck in the off state.