Copied from
https://discussions.apple.com/thread/2235949
“The startup sound uses the volume setting of the built-in speakers. So if you do not have external speakers connected, adjust the volume of the built-in speakers and that will change the volume of the startup sound.
If you have external speakers connected, Mac OS X maintains separate sound volume settings for the built-in speakers and "headphones" (used by the external speakers). So you can disconnect the external speakers, adjust the sound volume of the built-in speakers, and reconnect the external speakers. The external speakers will still use its own sound volume. However, the startup sound always comes from the built-in speakers, so that separate volume setting will be used.“