Here is a link from where you can download both SAPI4 and SAPI5. Once you installl SAPI4, add reference to its COM Component using the "Add Reference" dialog in your .NET project and use its interfaces.
Following are some helpful links on SAPI and its usage:
http://en.wikipedia.org/wiki/Microsoft_Speech_API
http://stackoverflow.com/questions/1460891/how-to-access-sapi-4-voices-from-net-framework-3-5