Finding Mac mail sounds plist I have been trying on and off for a fair while to edit the default Mail.app system sounds from their defaults. Ideally I'd like access to all my system sounds eventually, but Mail.app seems like a good start.

A link from LifeWire shows that you can duplicate your mail app and then overwrite the names but it seems clunky to me - does anyone know where the plist is that activates the sounds for Mail.app?
You can change the 'new message sound' in the mail preferences, as well as disable other sounds (send message 'swoosh' etc.).  Granular control outside of that isn't possible via the GUI.  See screenshot.

As per here Changing New Mail Sound in macOS Sierra, you can copy custom sounds to ~/Library/Sounds and they will show up as an option to select in Mail preferences.

One could then programmatically use the defaults command  to select your custom sound via Terminal or script if you run the following command from within the folder "~/Library/Containers/com.apple.mail/Data/Library/Preferences":  

defaults write com.apple.mail NewMessagesSoundName -string filename


where "filename" is the name of the file, without the extension, of what you copied to ~/Library/Sounds