The Speak command is demonstrated in Example 2 in the building digitalmods tutorial
http://www.mp3car.com/wiki/index.php...for_StreetDeck
I have just verified that it is working.
You might be missing a dependancy or your error could be coming from something else your doing, but programmatically, there is no setup that you have to do to use the Speak command and your are calling it correctly.
You can also call the Microsoft Speech SDK directly. Just download it from here:
http://www.microsoft.com/speech/techinfo/apioverview/