How to mute all sound in a page with JS?

This can easily be done in vanilla JS:

// Mute a singular HTML5 element
function muteMe(elem) {
    elem.muted = true;
    elem.pause();
}

// Try to mute all video and audio elements on the page
function mutePage() {
    var elems = document.querySelectorAll("video, audio");

    [].forEach.call(elems, function(elem) { muteMe(elem); });
}

or in ES6:

// Mute a singular HTML5 element
function muteMe(elem) {
    elem.muted = true;
    elem.pause();
}

// Try to mute all video and audio elements on the page
function mutePage() {
    document.querySelectorAll("video, audio").forEach( elem => muteMe(elem) );
}

This, of course, only works with <video> or <audio> elements, as items like Flash or JS initialized audio is impossible to restrict in general.