Apple Tea
it would be easier just to put a speaker inside a robot and then let it move its lips or mouth or whatever to the sound the speaker speaks.
what they do is like doing a step backwards.
http://www.popsci.com/gadgets/article/2010-04/freaky-robot-mouth-simulates-human-voices
The point is to accurately emulate the shape of a human mouth so that people with speech problems can learn from it. It's actually pretty neat.
But I'm guessing our robot overlords will just make do with speakers.