Hi @jemmyfrommelbourne -
We apologize for our late response.
About the notifications not appearing on your screen: Viber requires all Push Notification settings to be enabled. Otherwise, it will show the error message requesting to do so. In addition, another possible cause for this problem might be the fact that your iPhone is jailbroken/hacktivated, and that interferes with Push Notifications in your iPhone. Our support team might be able to assist you further with this issue - please thoroughly read this page in our official Help Desk: http://bit.ly/uRG1Dw
(At its bottom you may contact our Support Team if the solutions outlined in it were not sufficient).
About the sounds - Viber currently uses your phone's main volume settings, so if you want to make Viber's sounds silent, you need to use your iPhone's Silent button. In the future we plan to add volume settings specific for Viber 🙂