Maybe this was already discussed, but recently I found an interesting project:
ReadBeyond for audio ebooks; that is an ebook (epub3) with audio: you see the text and you hear a human (not robotic) voice (and the corresponding text is hightlightened). And a human voice is much more expressive than a robotic one.
It would be nice if Calibre could read this kind of epub (with its own audio, I mean).