I disagree a bit with your definition of fair use. Fair use is a bit a hodgepodge, I think IANAL, but the “four factors [0] #4 is the use’s limitation on creating value from the original work. It seems to me that creating captions from an audiobook in no way limits the copyright holder from selling more audiobooks.
There’s also a factor on profit/non-profit uses that doesn’t hold up with amazon providing it, but could if it’s not a sold feature or is provided through an OSS plugin.
The other factors are amount and nature. Since it’s the whole work that seems to go against fair use and the nature doesn’t seem to apply.
I didn't mean to imply my comment gave good coverage of the topic, just wanted to point out that "fair use" is focused on finding a balance between respecting copyright while protecting the freedom to make transformative works. It was definitely a nitpick comment on my part, with little real-world relevance to the original comment because it's inconceivable his personal-transcript would be legally challenged. And because it's so much work to manually-transcribe, to the point where he's likely to make mistakes and deliberate alterations and additions of his own, it'd be even more work to argue that it wasn't transformative, nevermind that it had any profit impact whatsoever.
There’s also a factor on profit/non-profit uses that doesn’t hold up with amazon providing it, but could if it’s not a sold feature or is provided through an OSS plugin.
The other factors are amount and nature. Since it’s the whole work that seems to go against fair use and the nature doesn’t seem to apply.
[0] https://en.wikipedia.org/wiki/Fair_use