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