I'm using USB headphones with a Mac Mini, and the lowest volume setting is too loud for most applications. Is there any way to increase the granularity (number of steps) in the volume control, or to apply some fractional factor to the volume level?
I had the same problem using a USB Plantronics headset.
I went to system preferences -> Sound -> Output -> Plantronics Headset -> Output Volume
Increase that, then for myself, I chose my default sound output to the internal speakers. Otherwise all the system sounds come through the headset.
Hope that is what you were looking for.
Just to be clear, if the poster is saying that the normal built-in volume control on Mac OS X doesn't allow him/her to lower the volume sufficiently, it may be possible to route the sound via a different sound driver (not the default system one).
You should be able to use an audio extension created in an app like Sound Flower.
To do this, in conjunction with the Audio MIDI Setup.app in your Utilities folder to control the 'master volume' in that extension, unless your particular USB speaker has that feature disabled.