It keeps the startup chime at a constant level (laud or soft, your choice)... For me, I have at a low level so that I won't wake up the sleeping baby...
Yup, as the poster above mentioned, check your volume in OS X. Your startup chime will sound at the same volume as that of OS X. So if you're muted, it will be muted. If you leave OS X at full volume, it will be at full volume.