I noticed that if I listen to music while I play (whether it's from the Music app or YouTube, etc.) the music volume is lower. When I switch back to the music app the volume level returns to normal. (iOS 10.3.1)
Yes the same thing happens to me, but it is the same in a lot of games. It's purely just the game puts an auto-quieter (does that make sense idk) on music. A quick fix is to pause the music and then play it again and it will go back to normal volume.