Windows 8 speech to text and text to speech API
I was able to get the Microsoft Speech Platform working on my Windows 7 laptop (both Voice Recognition and Text-to-Speech). You just need to install SDK and the runtime. You can also download additional Voice and Language packs. I would think it should work on Windows 8 as well. Here is a good sample on how to set it up to recognize some basic phrases like "Find restaurants near Seattle".
There are some new APIs for Windows 8.1:http://msdn.microsoft.com/en-us/library/windows/apps/windows.media.speechsynthesis.aspx
Text-To-Speech sample:http://code.msdn.microsoft.com/windowsapps/Speech-synthesis-sample-6e07b218
//build video on Channel9:http://channel9.msdn.com/Events/Build/2013/2-171
There are no Text to Speech or Speech to text libraries available in .Net for Windows 8 apps. System.Speech and such is not available. You will need to roll your own or find compatible 3rd party library.
I use Bing Translator service in my apps, but that has been removed from existance, they brought the text translator services into Azure, but did not bring speech over and they disabled the old website to get API keys for bing translator. Hadn't heard of Project Hawaii before will have to check it out.