There are some hints here to disable the startup chime, but none that I could find to control the startup chime volume in OS X. I have just found, though, at this site, a prefPane that controls the startup chime. It works fine for me on OS X 10.3.2.
The web page is in Japanese, but look for a link near the bottom of the page to download StartupSoundPrefPane100.dmg.gz. (as seen in the image at left). It works great to reduce the startup chime volume so I don't wake the entire family at night.
[robg adds: I downloaded this pane, but didn't install it as it wants to restart the system, and I'd rather not do that right now...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031220154651763