Turn off the notification sound at system level - Android Marshmallow

Automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn. To understand how MacroDroid works and required settings, my answer here

Your macro would look like this

• Macro : Kill Notification (name of macro )

Trigger: Application Launched ( choose from the drop down the apps for which you want the macro to run)

Action: Notification Sound ( select none, since you don't want any)

Constraints: None ( which means this macro always runs whenever chosen apps are launched)