Playing audio on Safari

I play audio with the following code:

var a = new Audio();

a.src = "xxx.mp3"; //.ogg if IE

a.play();

It works on different browsers including IE, firefox and chrome, but have no sound on Safari. I tried Safari 5.1 on PC and on iPod 4.3.3.

I tried .wav, .ogg, .mp3.

Anyone know how to solve this?