I'm running Mail 12.4 on a macOS 10.14.6 system. For some reason I no longer hear any new mail notifications when a new message arrives. As near as I can tell I have Mail configured correctly to play a notification for every new message:
So far, I've tried creating a rule that matched every message and would play a sound, but that didn't help. I've also confirmed that Mail is set to play sounds for its notifications in the Notifications system preferences.
I later discovered that, in fact, no alert sounds are playing on this system either, and that the issue is not just limited to Mail. As near as I can tell, the Sound Effects section of the Sound system preferences is configured correctly.
How do I fix this?