The app will have the audio muted if the mute toggle is enabled. On the iPhone, it's typically as simple as sliding this switch to the un-muted position.
On the iPad, it's a little more complicated, because Apple changed the mute slider to be a rotation lock when they released iOS4 last year. So, on the iPad, you may need to go into Settings, then General, and then change the "Use Side Switch to:" option to be "Mute", instead of lock rotation. After you do that, ensure the switch is toggled to the un-mute position and re-open the app, ensuring also that the volume is sufficiently adjusted. You should now have audio on the videos which include audio. On the videos which do not have audio, a small overlay appears at the beginning of each video to let you know that it doesn't have sound.
I hope this helps and thank you for purchasing our app.