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 > Sounds > Ringtones & Notifications > Phone Ringtone > Select a music file and use it as ringtone.