function decodeBase64ToArrayBuffer(base64String) {
        var len = (base64String.length / 4) * 3;
        var str = atob(base64String);
        var arrayBuffer = new ArrayBuffer(len);
        var bytes = new Uint8Array(arrayBuffer);

        for (var i = 0; i < len; i++) {
            bytes[i] = str.charCodeAt(i);
        }
        return bytes.buffer;
    }
Decodes the base 64 audio to ArrayBuffer used by Web Audio.
@param {string} base64String