How can I save Audible files as MP3s for my SanDisk player?

Is this possible? I'm very dependent on audiobooks and mostly get them from the library, downloading them on my Mac High Sierra and saving them on a SanDisk player. I mostly listen to them in bed for health reasons, so the player is more comfortable with tiny earbuds (if I fall asleep and roll over) and cheaper than a phone (i.e. if it fell out of bed).

The library just switched from Overdrive, which let me save the files indefinitely, to LIbby, which expects a 3-week return: of course this is "normal," but doesn't always work with my reading club schedules and listening speed. I'm also not going to create a black market business in audiobooks!

Today I got one for a reading club for free (I hope) from Audible and can listen to it on the phone and my Mac but not convert it to MP3: is this possible in any way? It shows up in my computer in Audible, and in ITunes as the name of a file and lots of storage and the cover photo but no sound.

Thank you!