static void androidUnmuteAudio() {
  androidUnmuteAudio(androidActivity());
}

static void androidUnmuteAudio(Activity androidContext) {
  AudioManager amanager = (AudioManager) androidContext.getSystemService(Context.AUDIO_SERVICE);
  amanager.setStreamMute(AudioManager.STREAM_NOTIFICATION, false);
  amanager.setStreamMute(AudioManager.STREAM_ALARM, false);
  amanager.setStreamMute(AudioManager.STREAM_MUSIC, false);
  amanager.setStreamMute(AudioManager.STREAM_RING, false);
  amanager.setStreamMute(AudioManager.STREAM_SYSTEM, false);
}

Began life as a copy of #1001275