I'd like to turn off the notification sound that occurs when
the screen is locked or power saving mode kicks in.

I searched this forum and 'google'd it', but all I found was
disabling all sounds.

These are global notification sounds. They can be configured through System Settings > Application and System Notifications > Manage Notifications. Applications tab. Check through each of the items in the drop down. Ones relating to Power Management or the Workspace will be particularly relevant to you.