Have not had this nuiscance in years, but post Catalina, getting good, sadistic shound effect. Today on lock / unlock (super duper). I did uncheck the 2 sound effects in preferences, and put slider to off, but still getting them, thanks.

I'm not sure if this is what you're looking for, but in System Prefs > Sound > Sound Effects you can either turn sound effects off or adjust their volume if they're turned on.

Thanks I turned both off and turned slider on off!