I love to changes sounds from the default once to custom once.
How then to change the default alert sound in mac OS Big Sur to a custom one. Easy!
Open Finder and click on top menu 'Go', next select Library
Now, find the 'Sounds' folder. Should be empty.
Drag and drop any sound that you want to use as an alert sound, very important the sound must be aiff extension.
Now open System Preferences and go to the Sounds tab, you will see your new sound :)