If your use case is to change the voice response, you could maybe tackle it by not using voice response, but using text-to-speech in your JS function. For example, if your command is "Show Voltage", and you want the response to be "Reading is X Volts" (where X is some data read from the device), you could try something like this:
* Define the voice command to run viewCtrl.showVoltage()
* Leave the Voice Response field in the Application Event blank
* In your code, have something like this:
$scope.showVoltage = function() {
// assume the IoT data is bound to the "voltage" app param
// show the Voltage label
$scope.setWidgetProp("voltage", "visible", true);
// set the label
$scope.setWidgetProp("voltage", "text", $scope.app.params["voltage"] + "V");
// say the result
$scope.app.speech.synthesizeSpeech({'text': 'Reading is ' + $scope.app.params["voltage"] + " volts"});
}