When you are watching video content and don't hear any sound when you think you should, please ensure that the your mute switch is not enabled on your device.
The mute switch is located on the side of the device close to the volume up/down button.
When enabled, all video sound will be muted.
Switching this off will re-enable video sound and allow you to adjust the volume. This is an Apple/hardware feature that the app adheres to.
It is worth noting that that some apps still have video sound even with the device muted (e.g. Safari/YouTube) but this is more than likely because the videos effectively 'take over' the full screen when playing. The custom video player in this app does not function in the same way.