1. Check if the volume is turned down.
2. Ensure that the sound is not switched off or mute mode is not activated.
3. Check the headphone jack for any dust or dirt and try to clean the jack.
4. Restart the device to see if the issue is resolved.
5. Check if the sound is working on other applications.
6. If none of the solutions above worked, then you could try factory resetting the device.