I haven't tested it myself, but the format seems to be DRM-free MP3, going by this thread:
https://www.mobileread.com/forums/sh...1&postcount=17
I am not interested in audiobooks, but the subscription scheme pricing is interesting, it makes buying the audiobook cheaper than buying the ebook version in many cases. I would sign up immediately if I could use the credits to buy ebooks instead of audiobooks.