The Bookseller recently hosted a seminar about audiobooks for 120 publishers, retailers and audio specialists. Media lawyer Robert Lands suggested having audio rights and book rights be separate, rather than having the book publishers nab them. Naturally the book publishers didn’t warm to that idea, but Lands had a good point: "Audiobooks in a way have far more in common with films than books in that there are actors, producers and a performance. There are a whole bundle of rights involved here that books just don't have."
There was agreement that one reason audio books haven’t caught on as well as they could is that agents and publishers aren’t necessarily the most technologically aware people. Agent Simon Trewin noted, “The book industry is very insular and cliquey, and often we like to deal with people we know. [Audiobooks] people are coming in from outside with new ideas, and we are not sure how to react."
If there's a book contract in your future, it might be worth trying to keep the audio book rights separate, especially if your publisher generally doesn't exploit such rights. Frankly, they'll resist, but at the very least you might be able to improve on the initial proposal.