Zebble makes it easy to listen to the user's voice to recognize the spoken words and use them for any kind of processing in your app.

Step 1: 

void StartButtonTapped()
{
      Device.SpeechRecognizer.Start(Listener);
}

void Listener(string heardWords)
{
     // TODO: Use the words as you please. 
     // The provided string may contain a single word; or a number of them.
     // For example, you can use a class-level StringBuilder class, add all heard words to it, and show the concatenated result on the screen.
}

void StopButtonTapped()
{
       Device.SpeechRecognizer.Stop();
}Make sure to stop the listener in your app at the right time. If you leave it open, the device will keep listening to the user and waste battery.

Note:  It's a good practice to Stop the listener in your module's Dispose() method.