How to avoid narrator and string to speech Overlapping in c#?

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C#

.NET

Process process;

    // Read Contetnt Mouse click event
    private SpeechSynthesizer readerSpeak;
    private void btnSpeakContent_Click(object sender, EventArgs e)
    {
        if(richTextBoxThanglish.Text.Trim() == string.Empty)
        {
            if (process.HasExited == false)
                process.Kill();
            MessageBoxWrapper.ShowWarning("Textbox is Empty.", "ABTS", "Textbox is Empty.");
            process = Process.Start("narrator.exe");
        }
        else
        {
            if (process.HasExited == false)
                process.Kill();

            readerSpeak = new SpeechSynthesizer();                
            readerSpeak.SpeakAsync(richTextBoxThanglish.Text.ToString());

            if (process.HasExited == true)
                process = Process.Start("narrator.exe");
        }
    }       

    private void btnStopSpeak_Click(object sender, EventArgs e)
    {
        if (process.HasExited == true)
            process = Process.Start("narrator.exe");

        if (!(readerSpeak == null))
        {                
            readerSpeak.Dispose();
        }
    }

Assume above program has two buttons call speakContent & stop_speak. I run narrator.exe process to get windows narrator. I do want to speek the richtextbox content without overlapping voice. I know about Synchronos speak, but it will pause my app until speakContent get finish. I need this with SpeakAsyn(). Is there anyway to add silent in the voice narrator? Actually i need that text speech, after 3sec. I had use "System.Threading.Thread.Sleep(3000);". But still text overlapping occurs, Means narrator & string to speech plays together.

Please help.