The problem is that wired headset it seems has priority higher then BT headset. So this snippet doesn't resolve the problem
AudioManager audioManager = (AudioManager)getApplicationContext().getSystemService(Context.AUDIO_SERVICE);
audioManager.setMode(AudioManager.MODE_IN_CALL); // also tried with AudioManager.MODE_IN_CALL
audioManager.setBluetoothScoOn(true);
audioManager.startBluetoothSco();
MediaPlayer mMediaPlayer = MediaPlayer.create(AudioTests.this, R.raw.test_cbr);
mMediaPlayer.start();