The sound played by the OLPC XO laptop when the laptop is powered on was provided by "The Edge" from the band U2. The sound was extensively equalised so that it would be accurate when played on XO speakers.
How to disable
Press the "Volume Down" key repeatedly on the keyboard whilst the start up sound is playing. You should hear it get quieter, and then mute. Next time you boot, it will not be heard.
How to enable
Press the "Volume Up" key a few times during boot. You should hear it return.
What does it mean if it cuts off too soon?
The sound automatically cuts off on XO-1.5 if you have pressed some of the keys that interrupt normal booting, such as Escape or the four game keys. This does not happen on XO-1.
On the XO-1 and XO-1.5, the startup sound is played into headphones or ear buds if there are any plugged in. To avoid surprise, unplug them. The equalisation for the XO speakers will cause the sound to be different when heard in headphones.
Headphones Socket Trivia
On the XO-1 plugging in headphones during the startup sound causes the sound to be immediately sent to the headphones, via an electrical switch in the socket. Software cannot prevent this hardware from operating.
On the XO-1.5 plugging in headphones during the startup sound does not redirect the sound to the headphones. This is a design decision in the software, since the headphone signal switching is software mediated.
Where is the source?
The file is kept in the images directory of the OpenFirmware sources for the OLPC XO. Here's a link: [Edge1-8k-EQ-Comp-Amp-Short.wav]
Playback via other output devices doesn't correctly reproduce the Edge's performance.