That dreadful Apple startup sound.

[BlackMonk]

Try this?  Source and applicable quote follows:

http://ubuntuforums.org/showthread.php?t=1398604&page=2

I know this thread has pretty much died out, but I found an answer on the ArchLinux Wiki at https://wiki.archlinux.org/index.php/MacBook --

1. Reboot with an OSX installation disc
2. Start the Terminal
3. Issue the command "# /usr/sbin/nvram SystemAudioVolume=%01"

I can confirm that this works on a late-2010 Macbook Air and Snow Leopard.

[BlackMonk]

And from here:

http://forum.soft32.com/linux/rid-starting-bong-G4-mac-mini-ftopict309614.html

I use the command nvsetvol from the package powerpc-utils on my ibook.
"nvsetvol 0" will turn it off, but I like "nvsetvol 1".