I have been using Audacity in conjunction with an M-Audio USB Fast Track device, on Windows, for quite some time. I recently got a Macbook and have installed Audacity to continue recording. However, when the Fast Track is connected, and the headphones are plugged into it so I can monitor the input levels, nothing is recorded to Audacity, even though Audacity recognizes the USB device is used as an input. I have doublechecked everything that could make this go awry (i.e. bit rate, whether or not the device is registered as an input, etc.) and it still does not record to Audacity. To clarify: when the USB device is connected, Audacity recognizes it as an input device, and I can hear the input signal through the device, but Audacity does not record the input signal.
[Note: this same problem occurs with Garageband, too. This leads me to believe that it might be either an internal problem within the computer hardware, or something wrong with the Fast Track (possibly something with the drivers)]
Apple (upper left) > System Preferences > Hardware > Sound > Input.
Is your device listed there and does the bouncing ball sound meter work when you make sound? This is the first stop in the computer. If it doesn’t get here, no other part of the system will get it including Audacity.
Most generic stereo USB devices just start working. No drivers needed. If the device is fancy with multiple sound channels and control schemes, then yes, you may need to install drivers.