System.Speech.dll is in netframework3.0,but in unity3 an error display:
NotImplementedException: The requested feature is not implemented. System.Type.GetTypeFromCLSID (Guid clsid) System.Speech.Internal.ObjectTokens.ObjectToken.CreateObjectFromToken[Object] (System.String name)
why NOT implemented? any suggestion is welcomed
Probably because it would increase the build of the player and Unity deemed that it would not be used enough for that increase to be worth it. It is basically a design decision, larger build or smaller build, but sacrifice speech recognition, which is used by a proportionately small part of the community.
There are plenty of namespaces that are not included in Unity's libraries. One example would be System.Windows.
Now, there is nothing stopping you from writing a plugin that works the same a System.Speech, but that is my guess as to why Unity does not include the library.