Can I lower the minimum volume on my Mac?
This only gives you one more notch below 1, but it might do as much as you need it to. Use the decrease volume button to lower the volume to 0. Then, press the Mute button once. This will let sound play, but at a lower volume than 1.

Also, you can change the volume in 1/4 increments with the following shortcuts:


⇧ + ⌃ + volume buttons in Snow Leopard
⇧ + option + volume buttons in Lion or later