RedditAutoUnmuteVideos.user.js

==UserScript==
// @name         Reddit Auto Unmute Videos
// @namespace    https://greasyfork.org/en/users/85671-jcunews
// @version      1.0.1
// @license      AGPLv3
// @author       jcunews
// @description  https://www.reddit.com/r/userscripts/comments/eizq63/is_there_a_userscript_to_automatically_unmute_all/
// @match        https://*.reddit.com/*
// @grant        none
// ==/UserScript==

(e => {
  //old layout
  document.querySelectorAll('button.volume[data-action="unmute"]').forEach(e => e.click());
  //new layout
  (new MutationObserver(rs => {
    rs.forEach(r => {
      r.addedNodes.forEach((n, b) => {
        if (n.querySelector && (n = b = n.querySelector("._2W9KLflUho3xU7KURAaHwH>button"))) {
          while (n = n.parentNode) {
            if (n.classList.contains("_3QOPLw8PZipW3i8kDxod81")) {
              if ((n = n.querySelector("video")) && n.muted) b.click();
              break
            }
          }
        }
      });
    });
  })).observe(document.body, {childList: true, subtree: true})
})()