Check your Source to ensure Audio is not disabled
Check Zone Settings to ensure Sound Output is Enabled and set to the correct Output
Ensure the correct Control Display is selected for the Zone, this tells Userful which device should play audio, and only one device in a Zone can be set as the Control Display at a time
If there is still no audio playback on your setup, test the device by connecting any pair of headphones to the 3.5mm audio port on the device
If you are using the Command & Control module, follow all the above steps in Userful Control Center, and also check each Source's audio settings in C&C.
Stopping and re-starting a Zone can resolve playback issues. If that does not resolve the issue, restarting all the Displays in that Zone can also help.