I like to listen to very quiet music on my MacBook, and my headphones/earbuds etc, seem to be very sensitive.  The volume doesn't go low enough for me, and turning down the software volume introduces nasty audio artifacts.  But I recently discovered a little trick to turn the volume down to 0!

To get a hidden quieter audio setting, use the volume button (F4) to turn the sound all the way down.  The audio will be cut off.  Then press the mute key (F3), and the audio will turn back on, but at a lower volume.