How do you check if music is playing by using a broadcast receiver?

You don't need a broadcast receiver for this - AudioManager is your friend:

AudioManager.isMusicActive() does the job you want, have a closer look here for details: AudioManager

here is an example:

AudioManager manager = (AudioManager)this.getSystemService(Context.AUDIO_SERVICE);
if(manager.isMusicActive())
 {
     // do something - or do it not
 }