<video id="video" autoplay loop muted>
<source src="/v.mp4" type="video/mp4" />
</video>
<button id="test">Звук</button>
<script>
var video = document.getElementById("video");
var volume = document.getElementById("test");
volume.addEventListener('click', function() {
video.muted = (video.muted == false) ? true : false;
});
</script>