I am lucky to have a friend that is a terrific voiceover artist, Kristi Burns. She generously agreed to narrate “Baker’s Dozen” and split the royalties.
Kristi used Spokenrealms.com to upload the audio book, but she also uses AudiobookBoom as a resources.
Once the book was ready for prime time, Spokenrealms posted it on their site, Audible, Googleplay, iTunes, etc.
How it works is that you pay for the narrator up front or split royalties. In most cases, the audio book service you use provides an engineer to edit and complete the audio book. You’ll have the opportunity to listen to the book and give notes before it's posted.
Amazon’s Audible also has a service that allows you to audition narrators called ACX. You can post a portion of your book and audition voices. Then you chose the one that best fits your narrative voice and make an agreement directly with the talent. I have friends that have used this service successfully.