In my project, I am playing music file in android media player by using the following code
MediaPlayer mPlayer = MediaPlayer.create(MyActivity.this, R.raw.my         
        
Inside the button click check for mediaPlayer.isPlaying(). This will return true if the media player is playing else false. 
So now with this, flag value you can make a if statement and switch to play or pause like this,
button.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
            if (mediaplayer.isPlaying()) {
                mediaplayer.pause();
            } else {
                mediaplayer.start();
            }
        }
    });