I keep hearing a sound at some irregular interval as I'm using my desktop (running Ubuntu 22.04.1).
Finally I notice that I had just received an email in Thunderbird

However, I've turned off notifications for Thunderbird over 1 week ago.

Confirmation
I sent an email to myself (from another machine - a mac) and waited.
I saw the email appear and I heard the sound play so this confirms it is actually Thunderbird playing the sound.
Additionally, there is no notification in the Ubuntu notification / menu area.

Greyed Out Sound Alert Option
I noticed that the sound alert option is still switched to the On side -- even though the over all alert option is turned off. In other words, you can see on the image above that the Sound Alerts option is over to the right as if it is on.
I turned on Thunderbird alerts but turned sound option off and sent myself another email. Settings look like the following now.

When the email arrived I heard a sound and saw a notification in the notifications area:

When I dropped the notification area I saw the email in there also (which with these settings are correct) but I should not hear the sound.
Why is this happening? Is this a bug in Thunderbird or in 22.04.1 settings?