The Guinea Pig in The Coal Mine

Here are three great tips:

- Use the option-shift key for fine-grain adjustments

- There is one more sound level below the lowest bar: go all the way down, which will automatically mute all sounds, and then un-mute

- Use the shift key to prevent a sound to be produced when changing the volume level via the keyboard (see http://www.tuaw.com/2010/02/25/mac-101-3-volume-control-tips/)

All the above can be done in the Sound preferences as well.