At first I thought Pokemon Go is the only issue, but today I installed Clash of Clans and it is the same situation: the game audio will try to go out through the Bluetooth channel but since there is no bluetooth device around, it is all mute.
If I plug in the earbuds, then I can hear something for both Pokemon Go and Clash of Clans. The other way is: if I find and turn on a portable Bluetooth speaker, or if I go to the car and use the bluetooth there.
But there should be a way to make these app go out through the iPhone's internal speakers. How can it be done? (if I turn off Bluetooth on the iPhone, it doesn't solve the problem).
P.S. the iPhone Music app doesn't have this problem. Only the apps do -- so I don't think it is apps' problem. I am using iOS 10.2.1 which is quite recent.
P.P.S. just as a test, I just installed Angry Birds Rio, and it is exactly the same with Pokemon Go, Clash of Clans: no audio. So I don't think it is the apps' problem.