[usdima]

Phone: Samsung Galaxy S4 Google Play Edition, Android 4.4.2

Settings/Notifications: Use system sounds: ON

Set your ring tone as custom ring tone which is .midi file. Please note that .midi is default extension for custom ring tones for Android 4.4, .mp3 can't be set as ring tone (at least, on my phone).

For incoming Viber call: do not answer and wait for other party to hang up.
Ring tone continues to play even after disconnect, the only way to stop it is to exit from Viber (or to terminate Viber using Task manager).

[ViberTeam]

Hi! Did you try to uninstall>reinstall Viber? Which Viber version do you have? (More>About)