Disable OS X startup sound How can I permanently disable the startup sound (or chime) in Yosemite?

(I am using a late-2014 13" macbook pro)

I already tried several options found here and there : 


installed StartNinja.app => did not work
installed Auto-Mute.app  => did not work
tried to install the old StartupSound prefPane => did not work
tried the command "sudo nvram SystemAudioVolume=%80" => volume does not change (I also tried other values than 80, such as 00, 01, or nothing)


NOTE: I know that it is possible to mute the next startup sound by turning the volume down, but this is not a permanent solution.
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.