mail.app – Mail in Mojave is no longer playing a notification sound when mail arrives

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:

Screenshot of Mail general preferences

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.

How do I fix this?