Sorry I missed this the other day.
I used to do it through some command line tools but Libation does it via an easy GUI. Log in (via Libation) to my Audible account and it downloads. Convert to .m4b, use the Overcast “upload” function to put the file in my account, and there you have it. All my audiobooks utilizing smart speed and voice boost.
Awesome I have a M4B tool already I didnt know about the upload feature.
It’s the primary reason I keep coming back to overcast, frankly. Easy website access to add and remove my own uploads is a big deal.
Now I know what I’m doing this weekend. Got a long list of “apps to try” 
I had never heard of iina and that looks quite impressive, thanks.
Ears seems like it was created for a pre-control center world where I can change audio sources in 2 clicks. Is that correct or am I missing some additional functionality?