I'm having some similar issues as others asking audio questions. I have a fixed layout EPUB that has a few sound effects files. I wanted these to be hidden for surprise discovery by kids. I made empty boxes overlying the images on the page into buttons that play the sound if touched. (Example touch an animal and hear a sound). That works perfect in Ibooks in the Mac, but on iPad, I can only get the sound to play if there is a visible button to push.
Is having an invisible button for audio not possible iPad?
What if I turned the image itself into a button?
I also have a sound effect that is triggered by an animation and plays on page load. This does seem to work correctly.