I’m seeing this question come up more often from authors:

The scenario – an audiobook only title. There is no eBook or print book to go along with it.

In this case, ACX (Audiobook creation exchange – distributes to Amazon and Audible) is not an option. They only allow you to assert titles that appear as eBooks in the Kindle store.

With Findaway Voices you can upload this audiobook, but you can’t distribute it to Amazon or Audible on this platform. But you can get the audiobook out everywhere else.