Can't disable mail notification sound

My Mac has recently started playing a sound whenever a new email message arrives. I never enabled this and want to turn it off, but I can't figure out how.

In the Mail app itself, I selected "None" as the New Message Sound, and un-checked the box that says "Play sounds for other mail actions".

I also checked the Notifications for the Mail app in the System Settings. I've seen other posts that tell you to go here and turn off the notification sound, but on my computer there is no option to do this. (I'm running MacOS Ventura, maybe it was present on previous versions).

How do I turn this off? Why did it turn on in the first place? It's only been doing this for a couple of days, but I've had my Mac for two years and never heard it before. The computer is a late-2020 Mac Mini (the M1 version).

Thanks,

Frank