[CANCUNATNIGHT]

Gracias a la pista que remite Agraham, he enlazado la librería de voz y ahora parece que funciona bien. Pronuncia toda la frase, reconoce vocales acentuadas, la ñ y demás caracteres españoles.
El código que he incluido en el enlazador "wrap" TTSwrap.dll es el siguiente:
------------------------------------------------------------------------
Thanks to Agraham for his trail, I have wrapped the TTS library and now, it seems to speak all the sentence, it admits accentuated vocal, the ñ character and others spanish symbols.
The code included for the class in the ttswrap.dll is the follow:

-----------------------------------------------------------------------
namespace Ttswrap
{
using System;
using System.Runtime.InteropServices;

public class esp : IDisposable
{
public esp()
{
Init_TTS();
}
[DllImport("TTS_spa.dll")]
private static extern void DeInit_TTS();
public void Dispose()
{
DeInit_TTS();
}
[DllImport("TTS_spa.dll")]
private static extern void Init_TTS();
[DllImport("TTS_spa.dll")]
private static extern int Say_TTS(byte[] texto);

public int Voz(string texto)
{
return Say_TTS(System.Text.UnicodeEncoding.Default.GetByt es(texto));
}
}
}
-------------------------------------------------------------------------

Happy New Year