You could make a QuickTime movie with just the audio you want, then put it in your System Folder with the name "Startup Movie."

This requires QuickTime 1.5 or newer.

Alternatively, you could also change the startup bong to the desired sound by buying one of DougG3's custom ROMs:

http://www.downtowndougbrown.com/programmable-mac-rom-simms/