setInput() {
    for (var i = 0; i < arguments.length; i++) {
      this.connect(arguments[i]);
    }
  }
Assign a parameter to be controlled by this envelope.
 If a p5.Sound object is given, then the p5.Envelope will control its
 output gain. If multiple inputs are provided, the env will
 control all of them.

 @method  setInput
 @for p5.Envelope
 @param  {Object} [...inputs]         A p5.sound object or
                               Web Audio Param.