How can I change the speed of spoken words, using CognitiveServices in Azure?

You can use the SpeechSynthesizer.Rate Property to Set or Get the speaking rate of the SpeechSynthesizer object.

It Returns the speaking rate of the SpeechSynthesizer object, from -10 through 10.

You can use the below Sample code to set a particular value for speaking rate in C#:

using System;  
using System.Speech.Synthesis;  

namespace SampleSynthesis  
{  
  class Program  
  {  

    static void Main(string[] args)  
    {  

      // Initialize a new instance of the SpeechSynthesizer.  
      SpeechSynthesizer synth = new SpeechSynthesizer();  

      // Set a value for the speaking rate.  
      synth.Rate = -2;  

      // Configure the audio output.   
      synth.SetOutputToDefaultAudioDevice();  

      // Speak a text string synchronously.  
      synth.Speak("This example speaks a string with the speaking rate set to -2.");  

      Console.WriteLine();  
      Console.WriteLine("Press any key to exit...");  
      Console.ReadKey();  
    }     
  }    
}