Well, I got SystemSound and added a startup sound which happens after I log in. I'm trying to find the sound that plays soon after you hit the power button. Any idea where that is and how to change it? If I find it and replace it with another sound but keep the name of the file the same and replace it, it should play my custom sound, right? I just need it's location.
That "blong" startup sound, right after you hit the power button, is in firmware/ROM and is not changeable, any more than the beep-beep, famous in older PCs or the silence in the newer PCs.
There is no alternate "bios" you can install.
You can however find a utility that makes the startup sound silent across normal reboots and startups.
Yet more info:
The blong occurs before nearly anything has had time to load. Way before the hard drive, so unless you want to hack your EFI settings (unknown territory. i don't even think i've ever heard of a developer working in that realm...) it is unlikely you will be able to edit that sound.
learn to love that sound as the sound of a healthy mac. if you turn it off, and your computer happens to go tits up, you will have lost the first port of diagnosis. if it's a problem that it might wake someone, keep a set of headphones near the mac, which you can plug in if you reboot at night, the sound will get bypassed into the headphones. to be honest, even just a minijack cut of the end of a set of headphones would work, as a totally effective and removable 'patch'.