Adding custom sound effects to macOS

In order to add custom sound effects to macOS, you need to copy the new sound files into the directory "~/Library/Sounds/" located in your home directory. The file name will be used to designate the new sound effect in the Sound panel of the System Preferences dialog. You can then select this new sound effect as the default alert sound.

Note that sound effect files need to be in the AIFF format; files in this format can be created via the iTunes application. Moreover, sample AIFF files can be found in the "/System/Library/Sounds/" which contains the built-in macOS sound effects.