MuteNetflixPreviews.user.js

// ==UserScript==
// @name         Mute Netflix Previews
// @description  Mutes Netflix auto-play previews
// @version      1.0.0
// @author       heavyLobster2
// @namespace    github.com/heavyLobster2
// @downloadURL  https://gist.github.com/heavyLobster2/600fe1bc7ba49f924eb8298eab4aca06/raw/MuteNetflixPreviews.user.js
// @match        *://www.netflix.com/*
// @grant        none
// @noframes
// ==/UserScript==
(function() {
    "use strict";
    
    function mutePreviews() {
      	var toggle = document.querySelector("div.global-supplemental-audio-toggle a");
        var icon = toggle.querySelector("svg.svg-icon");
        if (toggle && icon) {
            if (icon.classList.contains("svg-icon-audio-on")) {
                toggle.click();
            }
            return true;
        }
        return false;
    }
    
    (new MutationObserver(function (mutations, observer) {
        if (mutePreviews()) {
          	observer.disconnect();
        }
    })).observe(document, { subtree: true, childList: true});
})();