I think an important element to good audio retention is the intentions of the writer. If a book is written by a skilled writer, in a fluid, conversational manner, then a well-read dictation of it will likely be more enjoyable and carry higher retention.

There are some books (writing styles) that just don't translate to speech well. I believe that is more the writer than the content. A technically brilliant author can write in such a way that its impossible to follow word-for-word spoken, while his methods make perfect sense on the page.

Good topic OP- I have a few back-burner ideas involving audio components in web environments, and this area has always been of interest.