loud-youtube.user.js

// ==UserScript==
// @name         YouTube 音量加倍
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.youtube.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const videoList = document.getElementsByTagName('video');
    for (var v of videoList) {
        // create an audio context and hook up the video element as the source
        let audioCtx = new AudioContext();
        let source = audioCtx.createMediaElementSource(v);

        // create a gain node
        let gainNode = audioCtx.createGain();
        gainNode.gain.value = 10;
        source.connect(gainNode);

        // connect the gain node to an output destination
        gainNode.connect(audioCtx.destination);
    }
})();