The two decisions that matter most
One is creative and effectively permanent. The other is invisible and where most independent audiobooks fall over.
Casting, and whether it should be you
Age, accent and pace all shape how listeners read your characters. Memoir and business books often work better in your own voice, if you can hold a consistent read across hours. Fiction usually doesn't. Audition narrators on your actual pages, never on their demo reel, because a demo reel is someone at their best on material they chose.
Mastering to the numbers
Loudness, peak level, noise floor, room tone, bit rate. These are pass or fail checks, not preferences. It's the part nobody budgets for because you can't hear it working, only when it's missing.