Nach kurzer Google Recherche brauchst du das SDK5.1 gar nicht.
http://en.wikipedia.org/wiki/Microsoft_Speech_API
Zitat:
SAPI 5.3
This is the version of the
API that ships in Windows Vista together with new recognition and synthesis engines. As Windows Speech Recognition is now integrated into the operating system, the Speech
SDK and APIs are a part of the Windows
SDK.