Much like Nigel's amp that goes to 11, making it "...one louder," your Mac has a super-quiet mode. Macworld explains how to find it.
First, use your keyboard's volume keys to turn the volume all the way down (using the menu bar slider won't work). Next, press the "Mute" key. The speaker graphic will appear on screen and display the familiar "sound waves." Listen closely (or plug in your headphones) - your Mac is in super-quiet mode.
We're not sure when you'd want to do this, but now you can.