This seems to be what I've been using to disable it most recently: sudo nvram SystemAudioVolume=%80 https://discussions.apple.com/thread/3208783?start=120

This seems to be what I've been using to disable it most recently: sudo nvram SystemAudioVolume=%80 https://discussions.apple.com/thread/3208783?start=120&tstart=0 https://discussions.apple.com/thread/3208783?start=120&tstar...