To change the system's announcer of the time, under Mac OS X 10.5
(Leopard) :
1) Open the Apple Menu, located at the top left of the screen, and
select "System Preferences" from the drop-down menu.
2) Navigate to the clock tab and click "Customize Voice"
3) From the menu that pops up, choose a voice, or choose "Show more
voices" if the desired Cepstral voice is not visible.
4) Click "Play" to test the voice after selecting it, and the
resulting speech should sound like the announcer you have chosen.
I hope this helps...
-Paul