First, as mentioned above, please note that Viber does not offiically support Nexus 7, and this could unfortunately be the reason for the problem.
However, you may still check the following:
1. First, please try uninstalling and reinstalling Viber when connecting to a few different WiFi networks (in order to pinpoint the source of the problem). What are the results?
2. Also, please make sure the following ports are enabled ("forwarded") on your routers or firewalls:
TCP: 5242 + 4244
UDP: 5243 + 9785
(if you are not sure how to enable ports, you may contact your router manufacturer)
3. Try a few different earphones connected to the Nexus 7, and see if this helps.