"DomContentLoaded" doesn't work in my browser?

I wrote my html page as follows:

<div>
    <img src="Natural1.jpg" id="img1" >
 <audio src="sound.mp3" id="audio1" ></audio>
</div>

And my javascript file is like this:

function init(){
    audio1 = document.getElementById("audio1");
    var img1 = document.getElementById("img1");
    img1.addEventListener("click", imgClick, false);
}
function imgClick(){
    if(audio1.paused){
        audio1.play();
    }
    else{
        audio1.pause();
    }
}
document.addEventListener('DomContentLoaded', init, false);

I ran this in chrome12, the script first executed the method document.addEventListener, but it did not get into the init method, why? I tried the method attachEventinstead addEventListenerin IE8, but it still doesn't do it. what is wrong with my code?