I had my beta readers & family help me listen to auditions, and I followed my gut. As I say in the article, that first book was my narrator's first audiobook, too. But she was working with a really excellent production company, and that eased my mind. And her audition was just the best. She nailed the characters and the emotional tone. So, yeah, it's subjective--except that my beta readers all agreed.
We've had a horrible time with studios. They edited that part out. (I also didn't say the thing about the "chosen few" in there--geez, way to sound conceited--they put that in.) We did two books with the great studio, then ACX changed terms & the studio got out of the audiobook biz. Cue mad scramble for a new studio. My narrator found one she liked, and they were OK but hard to schedule, so she switched again for Book 4. DISASTER. She ended up having to teach herself to edit as the producer did a horrible job & then left us hanging. So with Book 5, she did it herself. I'm not completely satisfied, mostly by the speed, but she hasn't found anybody great to do it, and she does a very good job with the editing, so here we are.
But all those wrinkles--yeah, that's part of the reason I went with Audible for the second series. That and money. My narrator is pricey--I pay $380 PFH--and I write long, so it ends up being up to $5K per book. She also doesn't go that fast, mostly due to doing it herself, but also because she's an actress as well. So while I'm sure I've sold more audio because of having her--much more--I thought I'd do better with the power of Audible behind me, and the idea of not having to pay upfront AND an advance was huge. Not to mention the work of doing it yourself, because it IS work. A lot of work.
Working with Audible Studios so far has been great. I've been able to have a little input--was able to give my narrator preference, & they got me somebody I liked; they used my cover art; I got to give the narrator pronunciation and character notes. I also have a book that'll come out with Brilliance Audio for my tradpubbed book. On that one, it's invisible. I know they're doing it, but I have no input at all, and no advance as it's just part of the book deal. (I do get royalties, though.)