private void btnRecord_Click(object sender, RoutedEventArgs e)
{
   if (mic.State == MicrophoneState.Stopped)
   {
      mic.BufferDuration = TimeSpan.FromMilliseconds(100);
      data = new byte[mic.GetSampleSizeInBytes(mic.BufferDuration)];
      audio = new MemoryStream();
      mic.Start();
      this.PageTitle.Text = "Recording...";
      btnRecord.Content = "Stop";
   }
   else
   {
      mic.Stop();
      this.PageTitle.Text = "Playing....";
      btnRecord.Content = "Record";
      btnRecord.IsEnabled = false;
      PlayRecordedAudio();
      this.PageTitle.Text = "ready";
   }
}

Step 6: PlayRecordedAudio will be used to play the voice which has been recorded using Microphone. SoundEffect class converts memorystream into array which will played using Play metod.

Play method accepts below parameters. 

volume: Volume, ranging from 0.0f (silence) to 1.0f (full volume). 1.0f is full volume relative to SoundEffect.MasterVolume.

pitch: Pitch adjustment, ranging from -1.0f (down one octave) to 1.0f (up one octave). 0.0f is unity (normal) pitch.

pan: Panning, ranging from -1.0f (full left) to 1.0f (full right). 0.0f is centered.

private void PlayRecordedAudio()
{
   SoundEffect se = new SoundEffect(audio.ToArray(), mic.SampleRate, AudioChannels.Stereo);
   se.Play(1.0f, -1.0f, 0.0f);
   btnRecord.IsEnabled = true;
}

Step 7: Create XNADispatcheService class. FramworkDispatcher processes XNA framework event messages. Update method updates various framework components like power, state and media. IApplicationService interface has definition of two method StartService and StopService. StartService method will be used to Start the DispatcherTime and StopService will be used to Stop the DispatcheTimer. Tick event handler of DispathcerTimer will invoke frameworkDispatcherTimer_Tick periodically to update FrameworkDispatcher.

public class XNADispatcherService : IApplicationService
{
   private DispatcherTimer frameworkDispatcherTimer;
   public void StartService(ApplicationServiceContext context)
   {
      this.frameworkDispatcherTimer.Start();
   }

   public void StopService()
   {
      this.frameworkDispatcherTimer.Stop();
   }

   public XNADispatcherService()
   {
      this.frameworkDispatcherTimer = new DispatcherTimer();
      this.frameworkDispatcherTimer.Interval = TimeSpan.FromTicks(333333);
      this.frameworkDispatcherTimer.Tick += frameworkDispatcherTimer_Tick;
      FrameworkDispatcher.Update();
   }

   void frameworkDispatcherTimer_Tick(object sender, EventArgs e)
   {
      FrameworkDispatcher.Update();
   }
}

Step 8: We need to the namespace and objects marked in bold in App.xaml

<Application 
   x:Class="Windows_Phone_7___Microphone_Repeater.App"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
   xmlns:x=http://schemas.microsoft.com/winfx/2006/xaml
   xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
   xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
   xmlns:local="clr-namespace:Windows_Phone_7___Microphone_Repeater"><!-- Namespace of the application-->

   <!--Application Resources-->
   <Application.Resources>
   </Application.Resources>

   <Application.ApplicationLifetimeObjects>
      <!--Required object that handles lifetime events for the application-->
      <shell:PhoneApplicationService Launching="Application_Launching" Closing="Application_Closing" Activated="Application_Activated" Deactivated="Application_Deactivated"/>
      <local:XNADispatcherService/>
   </Application.ApplicationLifetimeObjects>
</Application>

Step 9: Run the application, press Record button and speak, press Stop button to repeat it back by Windows Phone 7.