Open WhatsApp and tap the three dots in the top right corner of the main interface
Click on "Settings" and then on the menu item "Notifications". At the top there is a button that allows you to enable / disable sounds on WhatsApp. Just below that you'll find "Notification Sound" - tap on it.
You will then see a list of the smartphone ringtones that you have on your device that can be used on WhatsApp.
Select and save settings