An interesting phenomena of personal interest has to do with “exceptional” people. These are students, children and adults that are bright, but may have a learning disability. It is well-known to psychologists, neurologists and school counselors (in my experience) that immersion in a subject by using more than one known “sense” is key to memory, retention, comprehension and recall.
I have a daughter and several grandchildren that are in the category of “exceptional” and we have been using bound books and and its audio book companion, simultaneously, to increase those skills necessary for success in school and life.
Although audio books have been popular for long car trips or commuting–it should be noted that in some cases, it’s not either/or, it’s necessary. In my opinion, any book worth publishing should have an audio companion.