This probably happens since the output voice language does not match your text. Try it out with a different output voice. Here is how you can get a list of all installed voices:
System.Collections.ObjectModel.ReadOnlyCollection<System.Speech.Synthesis.InstalledVoice> tmp = speech.GetInstalledVoices();
foreach (object MyVoice in tmp)
{
comboBox1.Items.Add((MyVoice as System.Speech.Synthesis.InstalledVoice).VoiceInfo.Name);
}
If no voice matches, you may want to install a new voice.