This little discussion got me curious about how hard it actually is to use COM interop with .NET. Using Visual Studio its just a right click on your dependencies, selecting 'Add COM Reference...' and choosing the library you want to use. I was then able to call SAPI very easily like this:

  var voice = new SpeechLib.SpVoiceClass();
  voice.Speak("Hello World!");

The documentation for SAPI could use some love though. The sample code that is supposed to be there is nowhere to be found.