Updated: Mar 2
Your audiobook narrator and producer will produce the full audiobook as the narrator and will also coordinate the final proofing, editing, and mastering of the audio files (I hire out for these services) which will be delivered to you in MP3 format for distribution via one of the audiobook marketplaces.
The most commonly used audiobook marketplaces are Findaway Voices, my preferred distributor, ACX, Audiobook Creation Exchange, which is owned by Amazon, which also owns audible.com the world's largest audiobook distributor, Audiobooks Unleashed, and Spoken Realms. There are many other smaller/boutique marketplaces that can help you "go wide" with distribution, (meaning not just on amazon platforms) but these are the main options for Indie/self published authors.
Once you have selected your marketplace distributor, they will provide a contract for both parties to sign that identifies the production timeline, the rate for the production, and any other pertinent details (all done through the marketplace.) The production phase starts with what is typically referred to as "the first 15 minutes" or the "extended sample." This can be any 15 minutes from the book, not necessarily the literal first 15 minutes. You ideally want to hear as many character voices/character names/pronunciations as possible in this sample to make sure your narrator understands the direction and 'voice' you want the book to have.
Once the First 15 have been approved, your narrator fully takes over the role of director and producer. Your input is very valuable, so that is why the First 15 is so important. Style changes/voice/pacing/ etc. are at the discretion of the narrator and cannot be changed after the First 15 have been approved.
There are lots of ways to go about payment, including royalty share, split royalties etc. and this link from Audiobooks Unleashed does a great job of describing all of those options. :) https://distribution.audiobooksunleashed.com/
Here is another really helpful link from Findaway Voices for authors: https://support.findawayvoices.com/portal/en/kb/articles/comprehensive-guide-to-au diobook-production