example.js

// e.g

var monitor = new Monitor(video);

setInterval(function() {
	if (monitor.getLevel() < 10) {
		console.log("No sound");
	}
}, 100);

monitor-video-sound.js

function Monitor(video) {
	var ctx = new webkitAudioContext();
	var src = ctx.createMediaElementSource(video);
	var anylz = ctx.createAnalyser();
	src.connect(anylz);
	alyz.connect(ctx.destination);
	this._bins = new Uint8Array(alyz.frequencyBinCount);
	this._alyz = anylz;
}

Monitor.prototype.getLevel = function() {
	var bins = this._bins;
	this._alyz.getByteFrequencyData(bins);
	var sum = 0;
	for (var i=0; i<bins.length; i++) {
		sum += parseFloat(bins[i]);
	}
	return sum / bins.length;
};