connected = false;
var gainNode;
function gain() {
  
  if (!connected) {
    var videoElement = document.querySelector("video");
    var audioCtx = new AudioContext();
    var source = audioCtx.createMediaElementSource(videoElement);
    gainNode = audioCtx.createGain();
    gainNode.gain.value = 3; /* triple the volume */ 
    source.connect(gainNode);gainNode.connect(audioCtx.destination);
    connected = true;
  } else {
    if (gainNode != undefined) {
    	gainNode.gain.value++;		
    }
  }
};