You cannot do so easily in managed code.
First you'll need to learn about
PInvoke and interop[
^].
Then you'll need to read
the manual[
^] and use interop to invoke SAPI5 functions in the Windows Speech API.
I'm not sure how much experience you've had with .NET so far but this may ba a bit ambitious. It took me a couple of years before I felt very comfortable with PInvoke and interop.