Copy over the releaseMediaPlayer() helper method from the NumbersActivity into the NumbersFragment.

Code for releaseMediaPlayer from NumbersActivity.java

/**
* Clean up the media player by releasing its resources.
*/
private void releaseMediaPlayer() {
   // If the media player is not null, then it may be currently playing a sound.
   if (mMediaPlayer != null) {
       // Regardless of the current state of the media player, release its resources
       // because we no longer need it.
       mMediaPlayer.release();

       // Set the media player back to null. For our code, we've decided that
       // setting the media player to null is an easy way to tell that the media player
       // is not configured to play an audio file at the moment.
       mMediaPlayer = null;

       // Regardless of whether or not we were granted audio focus, abandon it. This also
       // unregisters the AudioFocusChangeListener so we don't get anymore callbacks.
       mAudioManager.abandonAudioFocus(mOnAudioFocusChangeListener);
   }
}