A userscript that compresses YouTube audio so loud sections (such as intros that are at an average of 0dB when the video is at -6dB) aren't as awful.

YTLimiter.js

// ==UserScript==
// @name     		YouTube Audio Compressor/Limiter
// @description A somewhat passable solution to YouTubers who don't know how to look at a dB meter while editing their videos... (Makes very loud sections not as deafening!)
// @author      Tim (@ineedfocus)
// @include     https://www.youtube.com/*
// @include     http://www.youtube.com/*
// ==/UserScript==

//Apply compression to YouTube video
function fixAudio(element) {
  try{
    //Set up the audio context
    var myAudio = element;
    var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
    
    if(audioCtx != null && myAudio != null){
      var source = audioCtx.createMediaElementSource(myAudio);
      
      //Set up the audio processing     
      //Compressor:
      var compressor = audioCtx.createDynamicsCompressor();
      compressor.threshold.setValueAtTime(-24, audioCtx.currentTime); //This could probably go up a bit, but that'll make it less effective at canceling out loud sections
      compressor.knee.setValueAtTime(12, audioCtx.currentTime);
      compressor.ratio.setValueAtTime(4, audioCtx.currentTime); //Lower this to 2 or 3 if the pumping effect is a bit too strong for your tastes and you're okay with more loudness (1 = no compression)
      compressor.attack.setValueAtTime(0, audioCtx.currentTime); //Probably don't change this, it'll give you blasts of the original loud stuff before kicking in.
      compressor.release.setValueAtTime(0.15, audioCtx.currentTime);

     	//Limit volume:
      var gainNode = audioCtx.createGain();
      gainNode.gain.setValueAtTime(0.8, audioCtx.currentTime); //0 = mute, 1 is default
      
      
      //Source audio -> compressor -> gain
      source.connect(compressor);
      compressor.connect(gainNode);
      
      //Send processed audio to where it needs to be
      gainNode.connect(audioCtx.destination);
    }
  }catch(e){
  	console.error(e);
  }
}

//Wait for YouTube to insert their video element
function waitForElement(selector) {
  return new Promise(function(resolve, reject) {
    var element = document.querySelector(selector);

    if(element) {
      resolve(element);
      return;
    }

    var observer = new MutationObserver(function(mutations) {
      mutations.forEach(function(mutation) {
        var nodes = Array.from(mutation.addedNodes);
        for(var node of nodes) {
          if(node.matches && node.matches(selector)) {
            observer.disconnect();
            resolve(node);
            return;
          }
        };
      });
    });

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

//Do the thing!
waitForElement("video").then(function(element) {
  //console.log("Video element found, fixing audio!");
  fixAudio(element);
});

Just became aware of this and really appreciate this tool!
Only issue I have is that it seems Youtube's playback speed doesn't work with the script enabled for me