No YouTube Volume Normalization.user.js

// ==UserScript==
// @name         No YouTube Volume Normalization
// @namespace    https://gist.github.com/abec2304
// @match        https://www.youtube.com/*
// @match        https://music.youtube.com/*
// @grant        GM_addElement
// @version      2.74
// @author       abec2304
// @description  Enjoy YouTube videos at their true volume
// @run-at       document-start
// @allFrames    true
// ==/UserScript==

/* eslint-env browser, greasemonkey */

(function xvolnorm(pageScript, thisObj) {
    "use strict";

    var scriptId = "ytvolfix2";

    var logMessage = function(message) {
        console.debug(scriptId + "_injector: " + message);
    };

    var digestMessage = function(message, callback) {
        var msgBytes = new TextEncoder().encode(message);
        logMessage("attempting to hash script");
        window.crypto.subtle.digest("SHA-256", msgBytes).then(function(buffer) {
            var arr;
            var hex;

            if(typeof cloneInto !== typeof undefined) {
                // workaround for Firemonkey
                buffer = cloneInto(buffer, thisObj);
            }

            try {
                arr = Array.from(new Uint8Array(buffer));
                hex = arr.map(function(b) {
                    return b.toString(16).padStart(2, "0");
                }).join("");
                logMessage("obtained hash");
                callback(hex);
            } catch(_ignore) {
                logMessage("unable to convert hash data");
                callback("unknown");
            }
        });
    };

    var inject = function(hash) {
        var content = "(" + pageScript + ")('" + scriptId + "', '" + hash + "');";
        logMessage("preparing page script");
        if(document.head) {
            GM_addElement("script", {id: scriptId, textContent: content});
            logMessage("injected page script");
            return;
        }
        document.addEventListener("DOMContentLoaded", function() {
            GM_addElement("script", {id: scriptId, textContent: content});
            logMessage("injected page script (delayed)");
        });
    };

    if(typeof GM_addElement === typeof undefined) {
        window.GM_addElement = function(a, b) {
            var elem = document.createElement(a);
            Object.keys(b).forEach(function(key) {
                elem[key] = b[key];
            });
            document.head.appendChild(elem);
            return elem;
        };
        logMessage("defined addElement polyfill");
    }

    try {
        digestMessage(pageScript, inject);
    } catch(_ignore) {
        logMessage("unable to hash");
        inject("unknown");
    }
}(function(scriptId, hash) {
    "use strict";

    var logMessage = function(message) {
        console.debug(scriptId + ": " + message);
    };

    var _ignore = logMessage("page script called");

    var volumeColors = [
        "thistle",
        "plum",
        "orchid",
        "mediumorchid",
        "darkorchid",
        "darkviolet"
    ];

    var styleNum = 0;

    var addVolumeStyle = function(parent) {
        var repos = "position: absolute; left: 0; transform: translate(-100%)";
        var color = volumeColors[styleNum % volumeColors.length];
        var about = "No YouTube Volume Normalization #" + hash.slice(0, 16);

        var curStyle = parent.querySelector("style." + scriptId + "_style");
        if(curStyle) {
            logMessage("updating style");
        } else {
            curStyle = document.createElement("style");
            curStyle.className = scriptId + "_style";
            parent.appendChild(curStyle);
            logMessage("added style element");
        }
        curStyle.textContent = ".ytp-volume-slider-handle::after { background: " + color + "; " + repos + " }";
        curStyle.textContent += " .ytp-sfn-content::after { content: '" + about + "' }";
        curStyle.textContent += " ytmusic-nerd-stats::after { content: '" + about + "' }";
        styleNum += 1;
    };

    var setVolume = function(panel, video, setter) {
        var newVolume = panel.getAttribute("aria-valuenow") / 100;
        if(newVolume === video.lastVolume) {
            return;
        }
        video.lastVolume = newVolume;
        setter.call(video, newVolume);
    };

    var handleVideo = function(videoElem) {
        var parentL0;
        var parentL1;
        var desc;
        var setter;
        var volumePanel;

        parentL0 = videoElem.parentNode;
        if(!parentL0) {
            logMessage("video immediately detached from page " + videoElem.outerHTML);
            return;
        }

        parentL1 = parentL0.parentNode;
        if(!parentL1) {
            logMessage("video detached from page " + videoElem.outerHTML);
            return;
        }

        desc = Object.getOwnPropertyDescriptor(HTMLMediaElement.prototype, "volume");
        if(!desc) {
          logMessage("using archaic volume descriptor");
          desc = Object.getOwnPropertyDescriptor(videoElem, "volume");
        }

        setter = desc.set;

        volumePanel = parentL1.querySelector(".ytp-volume-panel");
        if(!volumePanel) {
            volumePanel = document.querySelector("ytmusic-player-bar #volume-slider #sliderBar");
            if(!volumePanel) {
                logMessage("abandoning - no associated volume panel");
                return;
            } else {
              logMessage("found music subdomain volume panel");
            }
        }

        addVolumeStyle(parentL1);

        Object.defineProperty(videoElem, "volume", {
            get: function() {
                logMessage("read of shadowed volume value");
                return 42;
            },
            set: function(_ignore) {
                var toCall = function() {
                    setVolume(volumePanel, videoElem, setter);
                };
                // slight delay to allow volume panel to update
                window.setTimeout(toCall, 5);
            }
        });
        logMessage("shadowed volume property");

        setVolume(volumePanel, videoElem, setter);
        logMessage("initial volume set");
    };

    var videoObserver;
    var intervalId;

    var existingVideos = document.querySelectorAll("video");
    logMessage("number of existing video elements = " + existingVideos.length);
    Array.prototype.forEach.call(existingVideos, handleVideo);

    videoObserver = new MutationObserver(function(records) {
        records.forEach(function(mutation) {
            Array.prototype.forEach.call(mutation.addedNodes, function(node) {
                if("VIDEO" === node.tagName) {
                    logMessage("observed a video element being added");
                    handleVideo(node);
                }
            });
        });
    });

    videoObserver.observe(document.documentElement, {childList: true, subtree: true});

    intervalId = window.setInterval(function ytvolfix2cleanup() {
        var scriptElem = document.getElementById(scriptId);
        if(!scriptElem) {
            logMessage("nothing found to clean up");
        } else {
            scriptElem.parentNode.removeChild(scriptElem);
            logMessage("cleaned up own script element");
        }
        clearInterval(intervalId);
    }, 1500);
}, this));

Forked this to change variables for new layout, fix was insanely simple
Youtube updated its code, now this script works for me. When its decides to use new UI, because its like switching 4th time in a month

As the above commenter mentioned, since the frontend code for the new UI was revised, there's no need for me to update the script at present.

Although there is a slight cosmetic issue with the display of the volume bar which I'll look at fixing.
-- cosmetic issue now fixed!