This is a common issue but it has a very simple solution: you have your ringer switch/button muted.
On the iPhone, it’s a switch and looks like this:
Make sure that the ringer switch is “ON” and not muted.
On newer iPads, the ringer switch is replaced by a software button in Control Center. It looks like this:
The iPad is muted when the button is lit up. Make sure it is not in order to hear the DDP YOGA NOW app audio.