Lowest volume is still too loud—how can I make it even lower? The lowest volume I can get by pressing the volume keys is still too loud when I'm wearing headphones. How can I make it even lower? 

I have a Mac Mini 2011 (5,1) running Mountain Lion.
Pressing ShiftOptionVolume Up/Volume Down allows you to adjust in quarter-box increments. You can also click the volume icon in the menu bar and fine-tune with the slider, or open the Sound panel in System Preferences and get a bigger slider there, which would allow for finer control.