Add a setVolume() function to affect all AudioContexts

mastervolume.js

// Based on https://github.com/notthetup/webaudio-volume-control/blob/master/mastervolume.js

(function() {
    window.setVolume = function(){};
    let origAudioContextConstructor = window.AudioContext;
    window.AudioContext = function(options) {
        let origAudioContext = new origAudioContextConstructor(options);
        let masterGain = origAudioContext.createGain();
        masterGain.connect(origAudioContext.destination);
        let origSetVolume = window.setVolume;
        window.setVolume = function(volume) {
            masterGain.gain.value = volume;
            origSetVolume(volume);
        };
        Object.defineProperty(origAudioContext, "destination", {
            value: masterGain
        });
        return origAudioContext;
    };
})();