Для того, чтобы воспроизвести свободный текст голосом можно воспользоваться библиотекой Microsoft Speech.

При создании проекта нужно подключить COM-объект Microsoft Speech Object

Пример реализции на C#

Исходный код примера: SpeechAPI.zip

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;
using System.Data;
using SpeechLib;
 
namespace SAPI5
{
      /// <summary>
      /// Summary description for SpeechForm.
      /// </summary>
      public class SpeechForm : System.Windows.Forms.Form
      {
 
            /// <summary>
            /// The main entry point for the application.
            /// </summary>
            [STAThread]
            static void Main() 
            {
                  Application.Run(new SpeechForm());
            }
 
            #region ...
            #endregion
 
            /// <summary>
            /// Required designer variable.
            /// </summary>
            private System.ComponentModel.Container components = null;
 
            SpVoice m_voice;
 
 
            #region SpeechForm
            public SpeechForm()
            {
                  InitializeComponent();
            }
            #endregion
 
            #region Dispose
            /// <summary>
            /// Clean up any resources being used.
            /// </summary>
            protected override void Dispose( bool disposing )
            {
                  if( disposing )
                  {
                        if (components != null) 
                        {
                             components.Dispose();
                        }
                  }
                  base.Dispose( disposing );
            }
            #endregion
 
            #region Windows Form Designer generated code
            endregion
 
            #region SpeechForm_Load
            /// <summary>
            /// SpeechForm_Load
            /// </summary>
            /// <param name="sender"></param>
            /// <param name="e"></param>
            private void SpeechForm_Load(object sender, System.EventArgs e)
            {
                  m_voice = new SpVoice();
                  m_voice.EndStream+=
            new _ISpeechVoiceEvents_EndStreamEventHandler(m_voice_EndStream);
 
                  foreach(ISpeechObjectToken t in m_voice.GetVoices("",""))
                  {
                        cbVoices.Items.Add(t.GetAttribute("Name"));
                  }
            }
            #endregion
 
            /// <summary>
            /// btnSpeak_Click
            /// </summary>
            /// <param name="sender"></param>
            /// <param name="e"></param>
            private void btnSpeak_Click(object sender, System.EventArgs e)
            {
                  if ((Convert.ToInt32(btnSpeak.Tag) == 0) && (sender != null))
                  {
                        btnSpeak.Tag  = 1;
                        btnSpeak.Text = "STOP";
 
                        try 
                        {
                             if (cbVoices.Text.Length >0)
                             {
                                   m_voice.Voice = m_voice.GetVoices(
                                   string.Format("Name={0}", cbVoices.Text), 
                                            "Language=409").Item(0);// 409
                             }
 
                       if (cbSaveToWave.Checked)
                       {
                           sfd = new SaveFileDialog();
                           sfd.Filter = 
                                     "All files (*.*)|*.*|wav files (*.wav)|*.wav";
                           sfd.Title = "Save to a wave file";
                           sfd.FilterIndex = 2;
                           sfd.RestoreDirectory = true;
                           if (sfd.ShowDialog()== DialogResult.OK) 
                            {
                              SpeechStreamFileMode SpFileMode =
                                    SpeechStreamFileMode.SSFMCreateForWrite;
                               SpFileStream SpFileStream = new SpFileStream();
                              SpFileStream.Open(sfd.FileName, SpFileMode, false);
                              m_voice.AudioOutputStream = SpFileStream;
                              m_voice.Speak(textBox1.Text, 
                                          SpeechVoiceSpeakFlags.SVSFlagsAsync);
                              m_voice.WaitUntilDone(Timeout.Infinite);
                              SpFileStream.Close();
                            }
                         }
                         else
                         {
                                   m_voice.Speak(textBox1.Text, 
                                        SpeechVoiceSpeakFlags.SVSFlagsAsync);
                             }
                        }
                        catch
                        {
                             MessageBox.Show("Speak error");
                        }
                  }
                  else 
                  {
                        btnSpeak.Tag = 0;
                        btnSpeak.Text = "SPEAK";
                        m_voice.Speak(null, (SpeechVoiceSpeakFlags)2);
                        m_voice.Resume();
                  }
            
            }
 
            private void btnExit_Click(object sender, System.EventArgs e)
            {
                  this.Close();
            }
 
      
 
            private void Volume_Scroll(object sender, System.EventArgs e)
            {
                  m_voice.Volume = Volume.Value;
            }
 
            private void Rate_Scroll(object sender, System.EventArgs e)
            {
                  m_voice.Rate   = Rate.Value;
            }
 
            /// <summary>
            /// m_voice_EndStream
            /// </summary>
            /// <param name="StreamNumber"></param>
            /// <param name="StreamPosition"></param>
            private void m_voice_EndStream(int StreamNumber, object StreamPosition)
            {
                  btnSpeak_Click(null, null);
            }
      }
}