Since Apple decided to split up iTunes to different places, it's harder to find your stuff. I used Audiobooks in iTunes in the past to store my own spoken stuff that I collected over the years.
So today I was wondering where my own audio books are, because I wanted to share one snipped with a friend. Found some articles but it's really related to the version of you macOS to find a proper place.
Because I know that the folder should be Audiobooks, I just used my terminal to search for it:
Go to your home directory (usually you should be there if you open a terminal)
find . -name Audiobooks
I found my content in this result:
In may case there are folders starting with a
just take a look in those folders and you will find your audio.