There are many reasons why a book might be unavailable.
Sometimes a book goes temporarily out of print - and sometimes no audio version has ever been recorded. Audible wants to give you the most complete selection we can and we'll keep adding series and filling in gaps as quickly as possible.
Sanderson is a master of his craft. With Kramer's performance the story is given a life of its own.
I've listened to the whole series so far, don' t know where Mr. Sanderson gets it all. Very entertaining though.