Like many people who have posted to this thread, I tried lots of the solutions that worked for some people, and they didn't work for me. I finally found an approach that worked for a while. Then, after a minor upgrade, it stopped working. I reread most of this thread again, and tried all the tips, without success. I eventually found a suggestion on another site, one of the dozens of sites that are discussing this problem. Subsequently, I found an almost identical procedure on a different Apple Support thread. I've noted the difference below. I don't think this approach has been listed here, but the thread is so long, it will be useful to new readers to have it listed near the end of the thread. It has worked for me. Perhaps it will work for some of those who haven't gotten success with the other techniques listed here. The technique is reversable, as indicated below. It may be necessary to run the command from an Admin account.
Open a Terminal window and type (or copy/paste) the following to turn off the startup noise:
sudo nvram SystemAudioVolume="%00"
[Another article says "%00" should be "%80".]
And to restore it to the original value type:
sudo nvram -d SystemAudioVolume
From: https://discussions.apple.com/thread/3632556?start=15&tstart=0 (scroll almost to the bottom of the page, and/or search for "Mr. Lion") and other sites.