Over the last year or so, Overdrive has been sunsetting their clients and pushing people to use the Libby web app leaving folks with older mp3 players, people who do a lot of offline listening, and people who just don’t like the Libby interface without a way to listen. I wrote a Firefox extension that lets you download the mp3s just like the old desktop clients used to do and it even will optionally parse the chapters for you. Hope someone finds it useful!
https://github.com/bookbonobo/libby-download-extension
Unfortunately it’s not possible in Chrome because of manifest v3. Mobile would be great, but I don’t have the expertise to make the UI work. If anyone reading this does and wants to make a PR that’d be great!