I thought you meant reading one thing while listing to a different audiobook. (I sometimes do this, and read your post while listening to Stephen King's Under the Dome, but I have to pause to respond. I can listen and read, but not listen and write.)

Anyway, I know a few people who often switch between reading the text and listening to the audio of a book. If your application seamless supports both reading and listening, whether it's simultaneously or switching from just one to just the other, it could be pretty useful for those who like to read sometimes and listen other times.

I think this misunderstands reading. Have you read a book this way? What did it feel like and what did you miss compared to reading in silence or to music?

Nevertheless, good luck with your idea and venture!