I don't see an option for them on the non-logged in page, but I buy a set of them, and then can use them how I want, not per month.
While the Netflix model is nice for residual content, 90% of the books I want are available on Audible - I'd rather have that selection than a more limited selection I could get in a subscription model.
I think some places do a subscription, though - Have you looked at Playster?
I know at least one author has written about this: http://www.antipope.org/charlie/blog-static/2015/06/cmap-why...
To say nothing of versions where it's just one person reading it straight through, though. Surely those are much cheaper.
https://goodereader.com/blog/e-book-news/scribd-reintroduces...
looks interesting.