Spare My Left Ear - Userscript to fix audio playing in only one ear

SpareMyLeftEar.user.js

// ==UserScript==
// @name         Spare My Left Ear
// @version      0.1
// @description  Fix audio playing in only one ear by changing all stereo audio on the page into mono
// @author       AjaxGb
// @include      *
// @run-at       context-menu
// ==/UserScript==

(function() {
    'use strict';

    const mediaSymbol = Symbol('spareMyLeftEarSource');

    class SpareMyLeftEar {
        constructor() {
            if (unsafeWindow.spareMyLeftEarManager) {
                throw new Error('Attempted to create a second Spare My Left Ear manager');
            }
            this.ctx = new AudioContext();
            this.ctx.destination.channelCount = 1;
            unsafeWindow.spareMyLeftEarManager = this;
        }

        static getInstance() {
            if (!unsafeWindow.spareMyLeftEarManager) {
                unsafeWindow.spareMyLeftEarManager = new SpareMyLeftEar();
            }
            return unsafeWindow.spareMyLeftEarManager;
        }

        addMedia(media) {
            if (media[mediaSymbol]) {
                return false;
            }
            console.log('Sparing left ear from:', media);
            const src = this.ctx.createMediaElementSource(media);
            src.connect(this.ctx.destination);
            media[mediaSymbol] = src;
            return true;
        }

        addAllMedia() {
            for (const media of document.querySelectorAll('video, audio')) {
                manager.addMedia(media);
            }
        }

        addAllMediaForever() {
            this.addAllMedia();
            if (!this.observer) {
                this.observer = new MutationObserver(() => this.addAllMedia());
                this.observer.observe(document.body, {
                    subtree: true,
                    childList: true,
                });
            }
        }
    }

    const manager = SpareMyLeftEar.getInstance();
    manager.addAllMediaForever();
})();