First please try turning your device off and on.
If the problem still persists, go to the 'My Page' and select 'Help center'. Then submit a request with the following information.
1. If this problem occurs when you are a Host, a Guest or a Participant.
2. If the volume of the device is on mute.
3. If there are no sounds coming from other apps as well.
4. Are you using earphones/headset or Bluetooth earphones/headset?