I've only ever transferred one book at a time via Calibre using the email@example.com
address. To the best of my knowledge you can only go one at a time.
I don't believe that there is any correlation between books received via USB or email and the voice reading. If this has happened for you I think it is purely coincidence.
Audio is the choice of the author of the book to decide whether they wish the audio to be withheld. By default audio is allowed unless the author specifically requests it not to be. Examples of this include authors who have also released the same book in Audio Book format and wish to get both sets of sales/royalties.