The chime is initiated by the logicboard's firmware, not the OS. That is why a Mac will still chime when no drives are present. The chime is within the OpenFirmware/EFI firmware on the logicboard. On a very few Macs it can be changed by changing the sound data a firmware update for the machine and reflashing the firmware. One of those few machines is the B&W PowerMac G3. No Intell Mac can have its boot chime changed. I believe the B&W G3 is the last machine that can have its boot chime changed.