You can setup jQuery's Sound Plugin. I'd auto-load the MP3, so there's very little wait between calling it, and hearing it. Then you would handle the response within your ajax call, and determine whether you'll play the sound or not.
$("#sound").sound({swf: url});
$("#sound").load(url);
$("#sound").play();
$("#sound").pause();
$("#sound").stop();
$("#sound").volume(0-100);
So running this every 4 minutes would look like:
$("#sound").load(url); // load our sound
setInterval(function(){
$.post("get-updates.php", {lastid:12}, function(response){
if (response.hasNewRecords)
$("#sound").play();
}, "JSON");
}, 240000);