Question

Is there any way to listen for volume changes with an Android service and react to that?

Btw: How does the Google music app allow the user to control the media volume even when the music is playing in the background?

Answer 1

Check out registerMediaButtonEventReceiver(ComponentName broadcastReceiver);

Define a BroadcastReceiver that handles ACTION_MEDIA_BUTTON. The recieved intent includes a single extra field, EXTRA_KEY_EVENT, containing the key event that caused the broadcast. You can use this key event to get which key was pressed.

EDIT:
This is just a sample code. syntax errors may be there.

// in onCreate of activity
registerMediaButtonEventReceiver(mediaReceiver ); 

// later somewhere in activity.
MediaButton_Receiver mediaReceiver = new MediaButton_Receiver();

class MediaButton_Receiver implements BroadcastReceiver {
     void onReceive(Intent intent) {

          KeyEvent ke = (KeyEvent)intent.getExtra(Intent.EXTRA_KEY_EVENT); 
          if (ke .getKeyCode() == KeyEvent.KEYCODE_VOLUME_DOWN) {

          }
          // Similarly other key codes .......
     } 
}

Unregister the receiver in onPause() or onStop()