According to the article How do I turn off in-app sounds?, you should be able to disable the sounds under the Media and Contacts entry at the bottom the of the Setting section. However on my Wife's Samsung Galaxy S9 (SM-G960U1) that section was missing:
Instead, I was able to open that section using Find Friends -> Contacts -> Settings. No idea why the section was missing from the main settings page, but any changes you make still seem to work once you can get into that section.
If that's the case. Edit the app with Apk Editor from Play Store. Delete the .ogg files compile it and install. If not so, check whether your phone has notification sound scheme at settings and notifications since you want other apps notifying.