Create a Custom Sound
If you would like to create a custom sound to notify you when you receive special mail, then check out these options:
Orator (freeware) is a small application used to convert typed text into a sound file. more info
Spoken Text App(freeware) is a very basic Applescript App to make text into a .aiff file with your current system voice. more info
Check out AT&T Natural Voices Text-to-Speech Engine demo more info
Samples: