There is no option to disable notification sound directly in the app. You can only turn notifications on and off.
App info settings directly in Android provide “Notifications” setting.
At first glance, it seems you can only enable or disable Default or Calls notifications with checkboxes, and to Allow notifications dot.
You have been deceived!
Both Default and Calls are clickable!
Click on Default and configure its Behavior – set it to “No sound” or “Show silently and minimize” to disable notification sounds.
…aaaaand the notification nightmare is over!