First please try turning your device off and on.
If the problem still persists, it would help us solve this problem for you and all other future users, if you could go to the Friends tab and click on the Settings icon.
Then click on Help center and submit a request with the answers to these questions and press send.
1. If this problem occurred with one specific user or with all users.
2. If this happened while meeting new people or while using the video call feature.
3. If the volume of the device is on mute.
4. If there are no sounds coming from other apps as well.
5. Are you using earphones/headset or Bluetooth earphones/headset?