If you use a set of external speakers with your Mac, you may notice that the chime you hear as your Mac starts up comes not from the external speakers but from your Mac's internal speakers.
If you'd like to adjust the volume of this startup chime -- or mute it altogether (not recommended) -- there is way to make this adjustment without changing the volume level of your external speakers.
OS X provides two volume sliders, one for the internal speakers and one for external speakers (or headphones) when attached. If you have speakers attached to the audio port of your Mac, the volume buttons on your keyboard adjust the volume of these external speakers. And if you go to System Preferences > Sound and click on the Output tab, you'll see that the volume slider at the bottom of the panel is for your external speakers as well.
Unplug your speakers, however, and this volume slider (and the volume buttons) instead controls the volume level for your internal speakers. This arrangement allows you to set the volume at different levels for external and internal speakers. So, you can lower the volume of the startup chime, plug back in your external speakers and continue to rock out at a high volume.
Although there is a check box to mute the sound of either the external or internal speakers, I would not recommend muting the internal speaker because the startup chime can alert when your Mac isn't feeling well. There is a handful of different startup tone patterns when your Mac is experiencing a problem, and you wouldn't want to miss such a warning.