Use Light Flow
. It will allow you to change notification sound, vibration, LED for a lot of apps including Viber. I use it on my phone for a few apps that either don't have a notification sound or don't give the option to change the default sound.
There is a free option also, but I am not sure if Viber is supported on the free version.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775