convert from text to speech

You already have good answers on text-to-speech on your previous question:

convert from text to speech[^]

So either you didn't look at the answers to your last question or you are actually asking about speech-to-text.

Google: "speech to text" or "speech recognition"

Nuance has been producing Dragon Naturally Speaking for years and they are still perfecting it.

There appear to be some free web services that will do it, but I suspect their accuracy may be poor.

Or perhaps you should investigate the new Kinect SDK which includes a sophisticated microphone array and speech recongition software.

Depends on what you want to do with the speech you are recongizing.

Text to speech with .NET is fairly easy: use System.Speech.Synthesis, use the class System.Speech.Synthesis.SpeechSynthesizer, see http://msdn.microsoft.com/en-us/library/system.speech.synthesis.aspx[^].

Speech recognition is much more difficult, see System.Speech.Recognition, http://msdn.microsoft.com/en-us/library/system.speech.recognition.aspx[^], use the either the class SpeechRecognitionEngine or SpeechRecognizer, see:
http://msdn.microsoft.com/en-us/library/system.speech.recognition.speechrecognitionengine.aspx[^],
http://msdn.microsoft.com/en-us/library/system.speech.recognition.speechrecognizer.aspx[^].

Code samples are available, see the above MDSN articles. The recognition classes normally work with the microphone, you won't need to do anything to prepare the microphone input. Some relatively difficult part is this: first class works only with Apartment State System.Threading.ApartmentState.MTA, second one — only with System.Threading.ApartmentState.STA, see http://msdn.microsoft.com/en-us/library/system.threading.apartmentstate%28v=VS.100%29.aspx[^]. Your UI library may work only in one of those apartment state (this is the case with WPF which requires STA), so you might need run the engine in a separate thread using System.Threading.Thread.SetApartmentState, see http://msdn.microsoft.com/en-us/library/system.threading.thread.aspx[^].

—SA