How to play / stop MediaPlayer using checkbox in preferences?

In this code, i made a preference check box with key "music" in xml folder.

   <?xml version="1.0" encoding="utf-8"?>
   <PreferenceScreen xmlns:android="http://ift.tt/nIICcg" >

    <CheckBoxPreference 
    android:title="Music"
    android:key="music"
    android:defaultValue="true"
    android:summary="Check/Uncheck this to switch on/off sound"
    />

    </PreferenceScreen> 

If this checkbox is checked , then my mediaplayer will start playing . If this is unchecked , it will not start. But the music is not stopping immediately after unchecking it.

What i want to implement is, if i uncheck the checkbox in preferences the music should stop immediatly

Below is the code i wrote in my MainActivity , i also used this code in onPause() method. But nothing works.

Hope you help me guys.. Thanks in advance..

        MediaPlayer song;
        song= MediaPlayer.create(this, R.raw.bg);
        song.setLooping(true);
        SharedPreferences prefs=PreferenceManager.getDefaultSharedPreferences(getBaseContext());
        Boolean music=prefs.getBoolean("music", true);
        if(music == true)
        song.start();