im a die-hard believeer in spaced repetition - i use it for everything (languages, books, code, etc.) it's basically impossible to use for audiobooks.
my bet is if you test yourself on a book you listened to two years ago you will not remember 99% of it, and if you read it it's probably more like 95% percent of it.
then again, everyone's mind is different, so maybe you retain voice more than audio
I have been using spaced repetition for the last 11 years to remember things, and I totally love it. I don't really like reading books, I prefer to listen to them. My solution for not being able to make flashcards while I'm reading a book is to listen to the audio book, when I hear a passage that I want to recall or think about, I write it down in a note taking app on my phone (I just use the default Notes app on my iphone), and then review my notes every couple of days and make flashcards of the stuff contained in notes.
I think I'm the opposite. When I was in school I was able to learn things much faster by attending the lecture compared to reading on my own.