I wrote a simple android app to broadcast voice over udp multicast. I also designed a simple encryption and advertising protocol to prevent unauthorized tapping and automatically discover new peers on the same channel. Here is the code:
https://github.com/hurui200320/Nojre

However, I intent to use this app when riding with my friend, now I can't figure out how to record audio from Bluetooth devices. The API is confusing. Someone suggest I should use AudioManager::startBluetoothSco(), but then I lost audio playback (aka hand-free mode), I clearly don't want that. I just want to record the audio from the bt mic. I tried AudioRecorder::setPreferredDevice, and it's not working too.

F*ck.