html-css

.container{
display:glex;
width:100%; //или нужная
}
.container div{
flex-grow: 1;
}

<video id="myaudio" controls="" loop="" autoplay="" name="media" preload="auto" hidden>
<source src="music.mp3" type="audio/mp3">
</video>
  <script>
    var audio = document.getElementById("myaudio");
  		audio.volume = 0.2;
  </script>