System sounds taken over App default sounds ie. Whatsapp, WeChat...

[Kassen Chu]

I just started using the Nexus 6P and installed all my usual apps. Having started to use my Whatsapp, WeChat, Instagram, etc, I noticed that all those apps are using the system default sounds. When I receive a message, it is no longer the classic tones that those apps come with. I've checked the notification tone in the apps itself, but I do not see the original ring tone. Only the system ones are available. Anyone can help pls?