Ensure the sound on your computer or laptop has been turned on and volume is turned up to an appropriate volume level.
iOS Mobile app:
Make sure the vibrate switch is turned so that vibrate is off. Once off vibrate, you can turn the volume up or down.
Android Mobile app:
Make sure the volume is turned up on the device, use the physical volume buttons if available or follow your manufactures guidelines for adjusting the volume on the device. Also ensure do not disturb or any manufacturer specific volume limiting features are not turned on for the device.