Phone: the microphone works again after Obeya was in the background

On the phone, the microphone button stopped responding once Obeya had been in the background (the phone asleep, or another app using the microphone), and only reloading the page brought it back. Each press now checks whether the microphone is still available and reopens it if the phone has taken it away, so push-to-talk works again as soon as you come back to Obeya.