Idk.js

// ==UserScript==
// @name        Volume to speed
// @author      Satan
// @namespace   youtube.com
// @description YouTube videos change playback speed based on the current volume. Fun.
// @include     https://www.youtube.com/watch?v=*
// @version     1.0
// @grant       none
// ==/UserScript==

'use strict'

function getVolume() {
  let slider = document.querySelector('.ytp-volume-panel')
  let val = parseInt(slider.getAttribute('aria-valuenow'))
  let isMuted = slider.getAttribute('aria-valuetext').match(/muted/) != null

  return isMuted ? 0 : val
}

function setSpeed(speed) {
  let video = document.querySelector('.video-stream')
  video.playbackRate = speed
}

// observe the volume slider
let lastVolume = getVolume()
setSpeed((100 - lastVolume) / 25)
let observer = new MutationObserver(mutations => {
  let volume = getVolume()
  if (volume == lastVolume)
    return
  else
    lastVolume = volume

 // console.log('Volume -> Speed userscript: volume changed to ' + volume + '%')
  setSpeed((100 - volume) / 25)
})

observer.observe(document.querySelector('.ytp-volume-panel'), {
  attributes: true,
  childList: false,
  characterData: false,
})

// console.log('Volume -> Speed userscript: observing volume')