From http://support.apple.com/kb/HT5068:
Movie and Audio Requirements
You cannot use media that have ".m4v" or ".m4p" file extensions.
It is recommended that you use H.264 video in an MPEG-4 container with an ".mp4" file extension.
For audio, it is recommended that you use AAC audio in an MPEG-4 container with an ".m4a" file extension.
Note: Video and audio media in HTML widget bundles are not DRM-protected.
Note: In some cases, movies and audio files in HTML widgets may not play in iBooks Author but will play in iBooks. If you are unable to play a movie or audio file in your widget in iBooks Author, try previewing on iPad. This should show you how the file will play in your completed book.
Have you tried testing your widget in Widget Tester (Preview -> Preview on Device)? Does it work there, but not when ultimately loaded into iBooks? If it doesn't work on Widget Tester, does the web inspector's console (Preview -> Show Web Inspector When Previewing) show anything? (You will need to select Preview again after enabling the web inspector.)
From our experiences, filenames using anything other than alphanumeric characters can be problematic.
If none of the above helps, I would recommend filing a new problem at bugreport.apple.com and attaching your working project.