Only one button will be responsible for everything, all videos will be with the same class. I tried, but it only mutes the sound from one video.

<video class="inst-video">
       <source src="preview1.mp4" type="video/mp4" />
</video>
<video class="inst-video">
       <source src="preview2.mp4" type="video/mp4" />
</video>
<video class="inst-video">
       <source src="preview3.mp4" type="video/mp4" />
</video>

<button onclick="enableMute()" id="toggleActionMute">MUTE</button>

let vid = document.querySelector(".inst-video");
function enableMute() { 
  vid.muted = true;
}