I mean, at $11 a year it doesn't sound like Audible is a critical part of your income, so why not create a gumroad account and sell directly from the authors and/or narrators website? It sounds like you could even offer a heavily discounted price and still come out ahead.
If enough producers come together this way you could even create some producer's co-op that can put up a basic webshop and account system.
All we need now is for someone to sit down and write an open source tool for it.
I'm a dreamer but I see a world where audiobook creators can create an account on a privately hosted instance, sort of like bookwyrm but with payment features, promote their work using ActivityPub in a federated way, and distribute the media p2p using something like peertube.
And consumers can hear the media in any of a number of clients.
Perhaps there are better players now, but if you're going to distribute the audio book on your own store, I think you need to be ready to recommend a good player for each OS your customer might be on. And it had better actually be a good player, or people will be upset with you, too.