Disable OS X startup sound
I would like to suggest a little script that I wrote to solve this problem without hacking the system. 

The idea is that the boot sound is not played if the Mac was muted before shutting it down. So this script basically mutes the Mac right before the shutdown and restore its previous state after login.

It should work on any version of OS X, and I tested it on my MacBooks with Yosemite.

You can find it on GitHub: https://github.com/teored90/nobootsound

The README tells you how to install the script. I hope this works for your system and I'd like to hear your feedback.