My questions are
1. I'm unsure about how to make it a good and useful product. What are some of the usecases? I can summarize blogs, books, etc, but what else?
2. What are the copyright implications here? Can I just take someone's voice and use it?
I think this might be better suited for public domain books from the past. If I could have The Count of Monte Cristo read by a Frenchman (in English), or Sherlock Holmes read by a British man with a Victorian accent, that would be cool.
Also another idea: audio versions of movie scripts. Select a character type for each role in the story, then automatically generate an audio version of the entire thing.
I know this because my suspension of disbelief gets somewhat broken when I recognize the voice actor in an animated feature. I imagine it would be similar with characters in a book.