If you are unable to hear the audio from the Digital Theatre app, first ensure that you have the most up to date version from the App Store. This should fix all known bugs with audio on your iPhone, iPad or iPod touch.
Once you have updated the app, if you are still having issues with audio from the app, ensure that your iPhone or iPod touch is not set to mute. The mute on iOS devices can be set in a number of ways with the mute set by either a hardware toggle switch near the top of your device, or through a software fix.
If the hardware toggle switch is controlling the screen rotation rather than audio, you can control the mute using the software switch. Press the home button twice in rapid succession. This will bring up a bar showing what apps are currently running. Scroll this list to the right and you will access the secondary mute switch, audio playback controls and your current audio source (normally iTunes).