Are you running two apps that have audio? I noticed this when listening to my podcast app Downcast or Spotify. If I start a game at the same time, it reduces the volume of each so that I have to go turn the volume back up. The annoying part is if I close the game, then the music or podcast returns to the old level and blasts my ears.
I guess it takes courage to run two audio apps at the same time in iOS10.