Some backstory: I use my MacBook at odd hours and don't want to disturb anybody with the loud startup chime. As you may well know, you can mute that by pressing the Mute key before shutdown with the headphones unplugged (otherwise, you'll just be muting the headphones). That's fine if you'll never need to use the int. speakers but I switch between them and my headphones during the day and sometimes I'll belatedly remember the need to mute but I've already plugged the headphones in.
So is there an alternative to mute the startup sound independent of having headphones plugged in or not?