Web Audio API Vol Adjust Based On Cursor X Pos

<audio controls autoplay src="http://mdn.github.io/media-source-buffer/viper.ogg" type="audio/ogg" controls autoplay>
</audio>
<pre>
<script>
var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var myAudio = document.querySelector('audio');
var pre = document.querySelector('pre');
var myScript = document.querySelector('script');
pre.innerHTML = document.body.innerText;
var osc=audioCtx.createOscillator();
var source = audioCtx.createMediaElementSource(myAudio);
var gainNode = audioCtx.createGain(source);
var CurX;
var WIDTH = window.innerHeight;
document.onmousemove = updatePage;
function updatePage(e) {
CurX = (window.Event) ? e.pageX : event.clientX + (document.documentElement.scrolLeft ? document.documentElement.scrollLeft : document.body.scrollLeft);
gainNode.gain.value = CurX/WIDTH;
myAudio.volume = CurX/WIDTH;
}
source.connect(gainNode);
gainNode.connect(audioCtx.destination);
</script>
</pre>