Hang on...
Since I don't use Audible I don't know what content they provide so perhaps they also offer performances, blogs, streamcasts or whatnot.
But why would a deaf person want closed captions for an audio version of a normal print book? Why can't they just read the book? I would have thought the experience is far superior than a machine translated (i.e. garbled) transcription of a condensed audio book.
Or perhaps this is a red herring and audible and trying to put search functions into their books or similar and the publishers are a bit odd about that.