error: Uncaught TypeError: Cannot set property 'innerHTML' of null [duplicate]

I keep on getting the error "Uncaught TypeError: Cannot set property 'innerHTML' of null" when I try to run this program in chrome. I do not now what I am doing wrong, and and any help would be appreciated.

<!DOCTYPE html>
    var x = "hey";
    var counter = 1;
    var button = document.getElementById("button");
    var div = document.getElementById("box");
    var text = document.getElementById("text");
    var sound = document.getElementById("sound");
    var array=[ "thanks for clicking", "keep on clicking", "click one more time", "why do you keep on clicking me?", "stop clicking me!"];
function thing(file){
    var y = array[Math.floor(Math.random() * array.length)];
        if (x == y){
            while (y == x){
                var y = array[Math.floor(Math.random() * array.length)];

    x = y;
sound.innerHTML = "<embed src=\""+file+"\"hidden=\"true\"autostart=\"true\" loop=\"false\"/>";

<form name="form1">
<input type="button" id="button" value="click" onClick="thing('sound.mp3')" />
    <input type="text" id="text" value="hey" />
<div id="sound">

Wait for the window to load:

window.onload = function()

or move your JS after the </body> tag.