Mail.app to play custom sounds?

I would like to play a custom sound file (.mp3) whenever an email arrives from a certain person.

In Mail.app Preferences, one can define a rule, but it's limited to choosing only a System sound to play.

I know that once upon a time Mail.app used to allow rule-based playing of user-supplied sound files. Did this feature get removed or morph into something unusable?

MacOS 10.14.6 Mojave

-Carl