When using vCastSender for ViewBoard casting for the first time, the Mac system will have a pop-up prompt to ask for audio authorization for "vCastSender". If the user does not agree, then there will be no sound when casting.
To manually allow access, go to:
1. System Preferences > Security & Privacy.
2. Select the Privacy panel.
3. Ensure that "vCastSender" is checked under Microphone.
NOTE: Ensure both the Mac and ViewBoard are not muted and their volume is turned on.