It all depends where the software has been installed: Windows, Android, iOS, because for each of the operating systems, the fix is different. For example, on Windows you will have to plug a MIC into the correct location and then change the audio settings. For Android, make sure that the sound is turned on. While you are in a conversation, press the Volume UP multiple times. Although the sound might be turned on, it's possible that you've turned it down for conversations only.
For iOS, go to Settings > Privacy > Viber and turn the slider on.
