Why some audio of the keynote widget don't work when I preview on the iPad??!!

Now I am making a book about helping people learning Chinese Pinyin (the Chinese phonics).

And I made some interactive exercise with keynote widget, that is you first see a vowel, and there are four colored boxes which have four different sounds, you can just click each box to hear what sound it is, and click the check mark to select your answer. one exercise includes about 7 this kind of questions.

The problem is it works perfectly on keynote, but some sounds just lost every time when I preview on iPad!!

The OS x is 10.8.4, the ibooks author is 2.0, the keynote is 5.3, and I am using the latest iMac and iPad 3.

And the audio file is .m4a. It seems everything is just fine, I feel very frustrated!! Is there something I missed,?

PS, I downloaded a free ibook, which called Korean Phonics, they used the same way to make this exercise, it works very well!! There must be a solution!!

Please let me know if you know how to solve this problem!!!

Thanks a lot!!!