I don't have a solution for you, but if you figure out what caused the sound to stop, you might share it on the thread here: https://community.verizonwireless.com/thread/767295. That's someone who has the start-up sound and wants to turn it off.
The "Droid" you hear on startup is usually connected to the Roaming sound. Tap on the menu key and tap on Settings>Sounds>Roaming. If there is no check mark in the check box next to Roaming, tap on it to check it. You should hear the "Droid" sound. To turn off the droid start up sound, just uncheck it.
I think that certain devices just don't have a roaming notification at all, because it's annoying and useless (XDA agrees). My device certainly doesn't have one, anyway:
I don't think the roaming notification has anything to do with the bootup animation.