Zencastr has a setting for Voip in the top bar of the project page. If this setting is turned ON, Zencastr will start a built-in VoIP session for you and your guests. This is the recommended way to use Zencastr. If this setting is turned OFF then you are responsible for using your own VoIP solution like Skype or Google Hangouts. If you cannot hear your guests, please ensure that the VoIP switch is ON.