How can I get voice recognition features into the Unity Game Engine? Is there a plug-in or a framework (hopefully freeware) that I could use? If so, do you have any ideas on how to install it? Also, how much of a problem would there be with background noises in the game interfering with the voice inputs into the game? Are there any examples of games on the market that use this? (besides for Spain 3D for the Torque Game Engine).
Unity3D doesn't have any direct support for voice recognition. You would need to write an extension (System.Speech.Recognition) or find some kind of plug-in to get you rolling. If you do end up going this direction you will lose out on the unity web player compatibility with your project.
I do not see any plug-ins that are already developed for this purpose.
Community Core Audio is looking pretty promising. It converts voice to text and is said to handle automatic speech recognition. The data is sent via the network through UDP so it would be easy to integrate into Unity. That is if you're not planning on using the webplayer.