Some library audio downloads are MP3 - they are DRM free. Most audiobook downloads are WMA. I don't believe the Kindle suprts WMA, and Calibre doesn't know anything about audio books. Kindle DOES support .AA books (Audible.com) natively, but you won't get those for free from your library. I usually keep one Audible book on my Kindle, just in case. But never more because Audible books are much larger than eBooks.
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