<!DOCTYPE html>
<html>
<script>
    function handleLoaded() {
        context = new (window.AudioContext || window.webkitAudioContext)();
        var myAudio = document.querySelector('audio');
        myAudio = new Audio();
        myAudio.src = "large-video-with-audio.mp4";
        var source = context.createMediaElementSource(myAudio);
        var gainNode = context.createGain();
        source.connect(gainNode);
        gainNode.connect(context.destination);

        testButton.onclick = () => {
            context.resume();
            myAudio.play();
        }
    }
</script>
<body onload=handleLoaded()>
    <input type="button" id="testButton" value="Click me"></input>
</body>
<html>