The comment, under which we have this discussion, gives it very specific context: where will you draw the line in the range of tools to transform audio for fair use.
Why would you consider pen+pencil and free app to transcribe audio to text different in terms of copyright infringement? Does Audible charge you more to use this feature?
What I see here is an attempt to dictate ways in which I can use book I already purchased - i.e. limit my rights.
What I also see: a bunch of companies that are trying to squeeze more juice from the market, which they are not even willing to serve in the first place, because they, being experts in the book market, couldn't figure out that market needs this feature, while a bunch of IT geeks, did.