As I understand it, the "bong" you hear on start-up is actually hard-coded into the hardware, since part of its function is to let the user know that the hardware is functioning correctly. Therefore, I don't think you can change it (although there are apps out there that will let you turn it off / mute it).
I don't want to get rid of it. Just want to know if there is a spot in the start up and shut down for more sounds. My wife has some Disney sounds at startup for her PC. I didn't know if Mac had the same features.