HTMLMediaElement.volume

The HTMLMediaElement.volume property sets the volume at which the media will be played. 

var volume = video.volume; //1

A double values must fall between 0 and 1, where 0 is effectively muted and 1 is the loudest possible value.

var obj = document.createElement('audio');
console.log(obj.volume); // 1
obj.volume = 0.75;

• The interface defining it, HTMLMediaElement.
• HTMLMediaElement.muted