How do I release MediaPlayer after it finishes playing a sound?

Try this

 voicePlayer.getMediaPlayer().setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
        @Override
        public void onCompletion(MediaPlayer mediaPlayer) {
            mediaPlayer.stop();
            if (mediaPlayer != null) {
                mediaPlayer.killMediaPlayer();
            }

        }
    });