That seems very silly.
And no, I don't think you can count audio books. The reading experience seems like an essential part of this.
Audio books are books as far as I'm concerned. I've seen a couple of people in the book threads really like them. As to the rules, this whole thread is a more democratic and honour based system. And not at all consistent, so in the last thread we left it for people to decide. People counted entire seasons of a TV show as a single entry {which is mind-blowing really). I'd only draw a line, if it were a short story. A short story does not a short novel make. But then again a short film, say a feature length documentary that is sixty minutes long, I would count, personally. Each to their own I'd say.
edit: As an example, what is the difference really between someone who consumed WAR and PEACE as an audio book and someone who read the damn thing? In terms of time they are both colossal entities and huge challenges.
edit: In addition, this list is about reading 50 books over the span of a year. And a year is a long time. If you have a book you want to read and haven't read in a long long time [maybe a film version comes out], I'd allow a reread. Books are wonderful things. Also graphic novels, I'd count, depending on length. Hell, if you read aloud to your two year old, over the span of a year, I'd count all of those tiny books [fifty plus in a year], into one book entry. Books are meant to be read, heard, read aloud, illustrated in some cases, read in one sitting, read over the span of many sleepless nights, re-read, read over the course of the year, and read to children as bedtime stories. The only thing that would not count in my opinion is to fool yourself. Reading non-fiction is okay [to count as an entry], but reading newspapers everyday is not. Reading fan-fiction novels online is okay [depending on length], reading a lot of short stories on forums is not [so no, don't count short-stories from neogaf writing challenges [merely note that you have read them, as a point of interest after the list].