Before Starting to Write a code for converting a text into Speech Recognition(Voice),you have to add Referecne(DownLoad if not available)Interop.SpeechLib.
1)Add the NameSpace as Using SpeechLib
2)Create an object of SpVoice class 
   SpVoice voice = new SpVoice();
3)call the Speak() method by using Spvoice class object as follows and pass the parameters in the Speak() 
    voice.Speak(String text,SpeechVoiceSpeakFlags Flags);
Lets Look the code:-

SpVoice voice = new SpVoice();

voice.Volume = 100;

voice.Speak(textBox1.Text, SpeechVoiceSpeakFlags.SVSFlagsAsync);



Place the above code in the Button Click event.
Add one TextBox ,write some text in TextBox and click on the button to convert you TextBox text into Speech Recognition(VOICE) as follows

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Text;

using System.Windows.Forms;

using SpeechLib;

namespace VoiceTest
{

    public partial class Form1 : Form
    {

        public Form1()
        {

            InitializeComponent();

        }

        private void btnSpeak_Click(object sender,EventArgse)
        {

            if (textBox1.Text != "")
            {

                SpVoice voice = new SpVoice();

                voice.Volume = 100;
     voice.Speak(textBox1.Text,SpeechVoiceSpeakFlags.SVSFlagsAsync);


            }

            else
            {
       MessageBox.Show("Please enter text for speech", "Text to Speech", MessageBoxButtons.OK, MessageBoxIcon.Information);

            }

        }

    }

}

Note:-If you didnt find the Interop.SpeechLib dll in your project,you can download from below attachments and add it.
Hope you enjoyed it !