As far as I understand, a vocoder won't make a human voice sound like Stephen Hawking. It only enables you to "tune" the incoming audio signal (vocal, guitar or whatever) to the notes played on a MIDI keyboard.
You need some propper speech synthesis software to sound like Mr Hawking. If anyone still has an Amiga 500 with a copy of Workbench, then there is an excellent text-to-speech function on that.
