How do I play a sound when an element changes, like SO Chat does?

I want a sound to play when an element changes on a page. I know how to do this, but I can't get it to play only on the first change, and don't do it later, until the user focuses the window (tab) and blurs it again.

My current code:

var notif = new Audio('http://cycle1500.com/sounds/infbego.wav');
if (window.innerHeight === window.outerHeight) {
  $(window).bind('DOMNodeInserted', function() {
      notif.play();
  });
}