In case you didn't know, Audible performs a strict quality control check on every audio file submitted. Its a two-part check and your audio MUST pass this or your audiobook won't be published, plain and simple.
In addition to working on dozens of audiobooks, I've also helped plenty of authors clean up their audio to pass ACX's strict standards. I've gotten all of them past this quality check on Audible, and I can help you too!
My rate is one gig per five minutes of finished audio.