I looked into a startup working on a similar problem - as long as the digital text and audio are for personal use, I think it should be ok (or at least, not worth going after). If it's possible to share with other users or post the output online, then I think there would be a problem - though unless it was being shared in the app, it's the distribution part that would probably attract adverse legal attention, not the scanning and ocr which has been around for a long time.
There's lots of precedent for format conversions, especially text to audio, being fair use. Accessibility lawsuits have set a lot of the rules of the road.