I use Calibre as well, as it's the best option out there. But I have been endlessly frustrated by the lack of metadata support for audiobook books. It wouldn't be that hard to tweak the amazon meta-data download plugin to fetch the audio version of a book, but I can't seem to generate any interest for this project. It's surprising since audiobooks are becoming more and more popular. I myself lack the programming skills to attempt this. But if Calibre was to support meta-download of audiobooks, it would be perfect for the task.
Edit: If someone feels comfortable with a bit of programming Kovid said it would be "easy enough" to tweak the amazon plugin to prioritize audiobooks instead of paper versions
https://www.mobileread.com/forums/sh...d.php?t=311524