Thanks Lager1. This problem started in early 2022. I always update, I've restarted endless times, I've even tried backup and restore. Nothing helps. I've just been though all notifications and turned sounds off. No joy.
Mail is directed to a number of folders by rules at the mail server. The sound occurs when a folder is opened, not an actual email. It only does it once for a specific email even if it's left unread. It only affects some emails but I can't work out why. I have a testing email subject "Test" contents "Test" carriage return "Sent from my iPhone" Even this causes the sound when the containing folder is opened. I would say about half received emails cause the problem. Rich.