This is a member of the Viber R&D Team!
@shto - Changing Viber's notification sound is not available at this point. :/
However, we'd be happy if you add your idea via: http://bit.ly/H2xu54
You can stop Viber sounds form your regular device settings - Viber mirrors the device setting.
Please feel free to let us know your thoughts, questions and comments, we'd be happy to assist.
the Viber Team