Hide YouTube videos (only listen to the audio)

youtubeVideoHider.user.js

// ==UserScript==
// @name         Youtube Video hider (to only listen to audio)
// @namespace    http://stackexchange.com/users/4337810/
// @version      1.1
// @description  A userscript that hides most of the video content on youtube, so you can listen rather than watch!
// @author       ᔕᖺᘎᕊ (http://stackexchange.com/users/4337810/)
// @match        *://*.youtube.com/*
// @run-at       document-end
// @grant        none
// ==/UserScript==

window.onload = function() {
    document.getElementById('player-api').style.height = '50px';
};

Perhaps modifying this as follows makes sense for clicks after window is loaded (or playlists):

// ==UserScript==
// @name         Youtube Video hider (to only listen to audio)
// @version      1.2
// @description  A userscript that hides most of the video content on youtube, so you can listen rather than watch!
// @match        *://*.youtube.com/*
// @run-at       document-end
// @grant        none
// ==/UserScript==

function hideVideo() {
  //console.log('hiding video..........');
  document.querySelector("div > video").style.opacity = '0';
}
setTimeout(hideVideo, 100);
// or
setInterval(hideVideo, 1000);