MacOS – Apple’s default desktop images stored

Any files located in the following locations will available to choose as alert sounds:

/System/Library/Sounds/
/Library/Sounds/
/Users/< username >/Library/Sounds/

(Note: By default the latter 2 folders may not exist, but if created and populated with files, you'll be able to choose them in the Sounds preference pane).

You might need to select a new folder of pictures and then back to the pictures you prefer. Deleting one of the plist files not all of them can sometimes leave the preference pane in a state of confusion.

Step one is to get some change, then re-enable the automated change part.