Steve Jobs wanted to make Mac advanced enough to run silently.
He used to tell Mac hardware designers to make the internal fans kick-in at a much higher temperature than contemporary PCs.
When you turn on a Mac, there is a loud bootup chime. If you want get rid of that chime and boot-up your Mac silently, you can do that with an app.
If you want to turn off your system startup chime, try StartUp Ninja. Just click on “Off” button that this app presents to you. If you want sound when you turn on your Mac, Boot up StartUp Ninja and click the On button.
StartUp Ninja works under OS X Lion and is a free download.
Source: Cult Of Mac