Getting video audio from another domain for visualisation

Question

On my web page I'm looking to get an audio level visualisation from a video source created on another domain, both of which I control. I am considering using the this library, which contains the following example code:

var myMeterElement = document.getElementById('my-peak-meter');
var myAudio = document.getElementById('my-audio');
var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var sourceNode = audioCtx.createMediaElementSource(myAudio);
sourceNode.connect(audioCtx.destination);
var meterNode = webAudioPeakMeter.createMeterNode(sourceNode, audioCtx);
webAudioPeakMeter.createMeter(myMeterElement, meterNode, {});
myAudio.addEventListener('play', function() {
  audioCtx.resume();
});

Because the source is from another domain, I cannot access the element directly with getElement. I understand the postMessage() function allows the sending cross-origin data, however am not sure how to apply it to a scenario like this, or whether this will even work. Any suggestions?