I have the Nexus 7 device as well and Viber is working just fine. The notifications are working and whenever I receive one, the tablet plays a sound. Access Viber and go trough settings because it's possible the notifications are not configured properly.
Secondly, press the Volume Up and expand the options then drag all the sliders to right.
If you still have issues with the application, try reinstalling it and then wait for notifications, because this could also be a bug.