Sound on complete for 'this.game.sound.play...'

[Hwang]

I have different sound file loaded through json and current using 'this.game.sound.play' method to play my sound.

I wanted it to do something when the sound is complete, but I can't find any tutorial on how to do it with that code,

so If I use the method on the example page like

var snd;

snd = game.add.audio('MySound'); 
snd.play();
snd.onStop.addOnce(onComplete, this);

it does works but I noticed the volume is louder comparae to the 1st code.
What seems to be the problem here?