[Paq Man]

How do you change the sound of the annoying "muuaaaaaahhh" when you turn on your computer?

[ManOfSteal]

Startup sounds are in the hardware ROM. You can't change them....

[CharlesS]

If you don't like them, you can plug a pair of headphones into the headphone jack.

If you just want to kill the startup chime, but leave the rest of the sound intact, you can get a pair of USB speakers, plug them into an empty USB port, and then plug a pair of headphones into the headphone jack. I'm guessing that the startup chime won't play through the speakers, since the driver won't be loaded yet, but after you boot into OS X, you'll get sound through them.

Just an idea...

[Mr. Blur]

as said, you cannot change the startup chime...but you can use Startupsound.prefPane to adjust or mute the volume.