KayMichigan Said:
"No New Mail sound notification in Mail program: Hi! I just got a new 2020 Mac Mini, and have noticed that I'm not getting any New Mail notification sound when I get new emails. I do get the whoosh sound when I send an email. All of the settings I can think of have been checked. From what I've read, this has been a common issue with Mail in Big Sur and was just wondering if they had figured out a fix for it yet. Thanks!"
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Reinstall the macOS:
The files went missing somehow. So, reinstall the macOS. Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be, respectively. Go Here: How to reinstall macOS from macOS Recovery - Apple Support.
Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall.