Just before the Christmas shutdown I was trying to load an ePub file that I had created in Pages to ithe iBookstore using iTunes Producer
The ePub contained a single audio file
I copied it to iTunes and synced with both my iPad and iPhone to test it - and on both devices it worked fine, the audio controls were displayed and worked.
When I tried to upload using iTunes Produce it would fail with an error message regarding the audio file (sorry I don't heve the exact message)
Tried various attempts using MP3 and AAC files (.m4v) - all would work on the devices, but not upload using producer.
In the end I turned the audio into a video file - with a simple title - uploaded that and it worked just fine.
The advantage of using a true audio file is that iBooks will multitask, you can turn to the next page, or even goto another application, and the audio will still be playing. With video it has to be the primary process.
Anyway - the book was a hit over Christmas, which was why I couldn't wait to get the issue fixed before publishing, but would like to try and understand the issue for next time