console.log('SilverDog filter is Running...');

let or_connect = AudioNode.prototype.connect; // NOTE: Keep original connect method
let filtrs = new Array; // NOTE: Store filters

function create_filter(context) {
	let biquadFilter = context.createBiquadFilter();

	biquadFilter.type = st_type;
	biquadFilter.frequency.value = st_freq;
	biquadFilter.Q.value = st_q;
	biquadFilter.gain.value = st_gain;

	return biquadFilter;
}

AudioNode.prototype.connect = function () {
	obj_name = arguments[0].toString();
	if (obj_name.indexOf('AudioDestinationNode') > -1) { // NOTE: If end node, add our filter in-between
		iltr = create_filter(arguments[0].context);
		filtrs.push(filtr); // NOTE: Keep filter in global array
		or_connect.apply(this, [filtrs[filtrs.length - 1], arguments[1], arguments[2]]);
		or_connect.apply(filtr, [arguments[0], arguments[1], arguments[2]]);
		console.log('Filter added.');
	} else {	
		or_connect.apply(this, arguments);
	}
};

let audio_elements = document.getElementsByTagName('audio');
let video_elements = document.getElementsByTagName('video');

if (audio_elements.length > 0 || video_elements.length > 0) {
	let c0nt3x7 = new (window.AudioContext || window.webkitAudioContext);
	let tmp_source;

	/*Loop for audio*/
	let len = audio_elements.length;
	for (let i = 0; i < len; i++) {
		tmp_source = c0nt3x7.createMediaElementSource(audio_elements[i]);
		tmp_source.connect(c0nt3x7.destination); // NOTE: Connect is overloaded already
	}

	/*Loop for video*/
	len = video_elements.length;
	for (let i = 0; i < len; i++) {
		tmp_source = c0nt3x7.createMediaElementSource(video_elements[i]);
		tmp_source.connect(c0nt3x7.destination); // NOTE: Connect is overloaded already
	}
}

console.log('SilverDog filter is Running...');

var or_connect = AudioNode.prototype.connect;
var filtrs = new Array;
var audio_elements = document.getElementsByTagName('audio');
var video_elements = document.getElementsByTagName('video');

function create_filter(context) {
	let biquadFilter = context.createBiquadFilter();

	biquadFilter.type = st_type;
	biquadFilter.frequency.value = st_freq;
	biquadFilter.Q.value = st_q;
	biquadFilter.gain.value = st_gain;

	return biquadFilter;
}

AudioNode.prototype.connect = function () {
	obj_name = arguments[0].toString();
	if (obj_name.indexOf('AudioDestinationNode') > -1) { // NOTE: If end node, add our filter in-between
		iltr = create_filter(arguments[0].context);
		filtrs.push(filtr); // NOTE: Keep filter in global array
		or_connect.apply(this, [filtrs[filtrs.length - 1], arguments[1], arguments[2]]);
		or_connect.apply(filtr, [arguments[0], arguments[1], arguments[2]]);
		console.log('Filter added.');
	} else {	
		or_connect.apply(this, arguments);
	}
};

if (audio_elements.length > 0 || video_elements.length > 0) {
	let c0nt3x7 = new (window.AudioContext || window.webkitAudioContext);
	let tmp_source;

	/*Loop for audio*/
	let len = audio_elements.length;
	for (let i = 0; i < len; i++) {
		tmp_source = c0nt3x7.createMediaElementSource(audio_elements[i]);
		tmp_source.connect(c0nt3x7.destination); // NOTE: Connect is overloaded already
	}

	/*Loop for video*/
	len = video_elements.length;
	for (let i = 0; i < len; i++) {
		tmp_source = c0nt3x7.createMediaElementSource(video_elements[i]);
		tmp_source.connect(c0nt3x7.destination); // NOTE: Connect is overloaded already
	}
}

Use `var` instead of `let` for global variables