Here's a simple trick to use your favorite music track as a ringtone in Viber. While Viber doesn't provide any option to select custom ringtones, it does allow you to make use of system sounds for notifications and calls. Basically if you are using a music track as default ringtone for your smartphone, you can also use it for Viber.
The first step is to define a custom ringtone for your smartphone. On Android, you simply have to go to Settings
> Ringtones & Notifications
> Phone Ringtone
> Select a music file and use it as ringtone.
Of course other methods exist, please read: How to change the ringtone of your Android phone
If you are using another mobile OS, please read:
How to create a custom ringtone for your iPhone on Mac OS.
How to create a custom ringtone for your iPhone
How to create a custom ringtone for Windows Phone
Once done, open the Viber app and tap on the Menu
key (3 horizontal bars located at the bottom right corner of your screen) > Settings
Go to the Notifications
Tick the Use system sounds
checkbox to use your phone's default ringtone:
NB: Change your phone's default ringtone again and the new setting will automatically apply in Viber.