The correct method to set the volume is audioobject.volume = VALUE
Here's a demo showing a similar setup as you want.
function changevolume(amount) {
var audioobject = document.getElementsByTagName("audio")[0];
audioobject.volume = amount;
}
<audio autoplay loop src="https://archive.org/download/animalsounds1/12wolveshowlfar.mp3"></audio>
<input type="range" id="vol" max="1" min="0" step="0.01" onchange="changevolume(this.value)" />