Quite a number of people experience a situation where Foliobook does not seem to be playing the sound from videos or songs from the iTunes library, but in reality the app is behaving correctly and the problem lies with the iPad settings.
The problem is caused by having the audio mute switch turned on, this is the black switch on the side of the iPad.
The problem is rendered more confusing because videos do play with sound from Apple's video app, even when the mute switch is on.
Note that this can be made even more confusing because its possible in the iPad settings app to reassign the mode of the black switch from audio to orientation. If this is done while the black switch is muted, then the sound becomes effectively locked off until the black switch is reassigned as an audio switch.
To check this, open the iPad settings and find General and 'Use Side Switch To' to set this to 'Mute'
Then open Foliobook and toggle the side switch on the iPad while playing a video or music. This should result in the app playing sound correctly. Once you have determined this you can readjust the side switch as you need, and return to using the software mute switch in the system tray (revealed when you double tap fast on the round button and swipe to the full left).