amplify-sound.js

function amplifyMedia(mediaElem, multiplier) {
  var context = new (window.AudioContext || window.webkitAudioContext),
      result = {
        context: context,
        source: context.createMediaElementSource(mediaElem),
        gain: context.createGain(),
        media: mediaElem,
        amplify: function(multiplier) { result.gain.gain.value = multiplier; },
        getAmpLevel: function() { return result.gain.gain.value; }
      };
  result.source.connect(result.gain);
  result.gain.connect(context.destination);
  result.amplify(multiplier);
  return result;
}

let [video] = document.getElementsByTagName('video');
let multiplier = 4;

amplifyMedia(video, multiplier);