edit login screen?

I recently began editing my mac system sounds and looks. For the log in screen, i would like to add a sound (.aiff), to play each time the use types in the password incorrect (box shacks, sound plays) I though it would be cool to put a computer voice in there say something like "The password you typed is incorrect. Please try again." something like that...Any ideas how to do that?

 

No. I have managed to edit the text, picture, and background in the login screen but havnt found a way to add a sound. I know where that stuff is in the syste, but im guessing i have to create something to add a custom sound...