Temporarily Disable Boot Chime on Mountain Lion

I recently purchased a 2012 13" Macbook Air with Mountain Lion installed for university. I was looking online for a way to disable the boot chime during startup besides muting the volume every time I shut the system down (I am a little disappointed there are no options to adjust the chime in the system preferences, but what can you do). Most of the suggestions I found were to install 3rd party apps or mess around with scripts in terminal. However, I found a few people saying that you could temporarily disable the chime by holding the mute button while booting. This would be perfect for me, as I only want the chime muted in certain places (like the library where people are studying). However, I tried this solution and it does not seem to work on Mountain Lion. My system is fully updated to 10.8.2. Did they remove this feature in Mountain Lion, or did it even exist in the first place? If it is still possible, could you please let me know what keys to hold while booting. I tried various combinations with command and option, but to no avail. Thank you.