You might find the first answer interesting here: http://dsp.stackexchange.com/questions/7833/how-to-mimic-cop...

Looks like there's some interesting research on Voice Conversion out there, but I'm not knowledgeable enough to comment on how close we are to seeing it as an actual product. This pdf (from 10 years ago) seems interesting, though. Hmmm: http://nlp.lsi.upc.edu/papers/thesis_helenca.pdf

There's a Scottish company that does something like this but it's boutique and requires a large corpus. They recreated Roger Ebert's voice after he lost his jaw but it was only possible because of his vast recorded collection of movie reviews to draw on for phonemes.