Why won't my sound files play in preview?

I have created a Keynote slideshow for a new course on an iMac, and I am trying to preview it in iTunes U on an iPad. For each slide of the show I imported a sound file of my lecture, which I recorded in Garageband. When I open iTunes U on my iPad, I find the course just fine. Then I find the new POSTS and click on one of them. The slide show opens nicely on the iPad, but there is no sound and no sound on-off button.

Where did my sound files go? How do I access them on iTunes U?

Thanks for the help.

Tromba