My (own) company is producing EPUB 3 Audio-eBooks for an Italian audiobook publisher. More than ~1K users bought them already, and I think that having text+audio (not TTS, but "real" human narration) is a pretty interesting experience.
But, of course, someone might prefer just reading the text. Or just listening to the narration.
BTW, you can download a free EPUB3 Audio-eBook of "The Curious Case of Benjamin Button" by F. Scott Fitzgerald here:
http://www.smuuks.it/index.php/en/pr...enjamin-button