I had my iPhone output music to my car's Bluetooth, and so Pokemon Go also output the audio to my car as well.
But when I got home, the Pokemon Go audio was still tied to Bluetooth. I had to use a Bluetooth device at home, or plug in the headphone, in order to hear anything.
Even if I reintalled Pokemon Go (by turning off Bluetooth on the iPhone, then rebooting the iPhone, uninstalling Pokemon Go, and rebooting iPhone, and finally reinstalling Pokemon Go), the Pokemon Go audio would still be tied to Bluetooth.
Is there any way to make it output the audio through the iPhone speakers again?