Transformative alone isn't enough for a fair use defence. Nor is it required. It's simply one of the many factors a judge will take into account.

But it was an important factor in the google books case.

One of the other key factors is how it impacts potential sales of the original work. Turning it into an audiobook might be transformative, but when you sell access to it people will buy your audiobook instead of the original book. So it's almost certainly not fair use.

In the google books case, google scanned the books but didn't distribute the content of the books to the user. They only distributed the transformed ability to search books to users. The sales of the books weren't impacted negatively, because the user still had to acquire a copy of the book from somewhere else if they wanted to read the whole work. In fact, google books arguable increases sales of the original work in some circumstances.