Hi,You may need to install SAPI4 alongside SAPI5 depending on the voice you want to use. Some do not work with SAPI5 even when they appear to be compatible.
Take a look at these couple of links which tell you where to download SAPI4 and other information related to TTS.
http://www.reallusion.com/crazytalk/ct_tts.asp
http://www.microsoft.com/speech/speech2007/default.mspx