Sometimes I read audiobooks. Sometimes I listen to text books.
Here's the thing: occasionally I need to see how a word is spelled. Other times I need to hear it pronounced. Sometimes I'm on a bus, and sometimes I don't have my headphones. So why isn't there an application that tracks progress through both ePub and audiobook formats?
This would be fairly simple, I think. It's a metadata problem, basically. You would simply treat an ePub file as a video caption file. It simply needs to be mapped to the audio, and the rest is trivial!
Of course, the process of mapping audio to text may need some data-entry types. Unless someone could write some code to sit on a server running speech-to-text. Even it if only mapped words with a high level of certainty, I'm sure this would work well for the most part.
Getting publishers to agree on how to bundle audiobooks and digital text is the real stumbling block I assume.