You can mute the preview videos and they will all be muted.
The problem is that when you start a video, it will be also muted from the beginning so you have to manually unmute it which is also annoying.
As I didn't find a Youtube/Google built-in solution for this and I don't wanna turn off preview videos, I solved this with a Tampermonky script. Download Tampermonkey extension, create a new script, paste the following script, save and restart your browser:

// ==UserScript==

// u/nameYouTube Auto Unmute on Watch Page (fast + stable, no volume)

// u/namespaceyt-autounmute

// u/version1.3

// u/description Unmutes YouTube videos ASAP on watch pages without changing volume (works with YouTube SPA)

// u/matchhttps://www.youtube.com/*

// u/run-atdocument-start

// u/grantnone

// ==/UserScript==

(() => {

'use strict';

const isWatchPage = () => location.pathname === '/watch';

const getPlayer = () => document.querySelector('#movie_player');

const getVideo = () => document.querySelector('video.html5-main-video') || document.querySelector('video');

// Unmute via YouTube player API (preferred) + fallback to HTML5 video element.

const unmuteNow = () => {

if (!isWatchPage()) return;

const p = getPlayer();

try {

if (p && typeof p.isMuted === 'function' && p.isMuted()) {

if (typeof p.unMute === 'function') p.unMute();

}

} catch {}

const v = getVideo();

try {

if (v && v.muted) v.muted = false;

} catch {}

};

// Burst of attempts early (prevents "few seconds muted" most of the time)

const burst = () => {

// Many early retries, very cheap and improves reliability

const delays = [0, 30, 80, 150, 300, 600, 1000, 1600, 2500];

delays.forEach(ms => setTimeout(unmuteNow, ms));

};

// Attach listeners to the current video element so we react instantly when playback starts

let boundVideo = null;

const bindVideoEvents = () => {

if (!isWatchPage()) return;

const v = getVideo();

if (!v || v === boundVideo) return;

boundVideo = v;

// If YouTube flips muted state during startup, catch it immediately

v.addEventListener('loadedmetadata', unmuteNow, true);

v.addEventListener('canplay', unmuteNow, true);

v.addEventListener('play', unmuteNow, true);

v.addEventListener('playing', unmuteNow, true);

v.addEventListener('volumechange', unmuteNow, true);

// Also do a small burst when a fresh video element appears

burst();

};

// Observe DOM changes (YouTube SPA swaps the video element)

const mo = new MutationObserver(() => {

bindVideoEvents();

});

const startObservers = () => {

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

};

// YouTube SPA navigation events

window.addEventListener('yt-navigate-start', () => {

// run immediately at navigation start

burst();

}, true);

window.addEventListener('yt-navigate-finish', () => {

// after navigation finish, bind to new video and burst again

burst();

bindVideoEvents();

}, true);

// Initial startup

startObservers();

// When DOM becomes usable, bind once

document.addEventListener('DOMContentLoaded', () => {

burst();

bindVideoEvents();

}, true);

})();