Using Arduino as USB Device?
LUFA is used for the USB to serial firmware on the separate USB-serial bridge AVR found on Uno and newer Megas. The firmware is compiled using a supplied makefile. Otherwise LUFA isn't used by Arduino.

But LUFA is probably the right choice if you want to make a USB audio device. It comes with examples of both input and output audio devices. But you will have to compile using the supplied makefile, unless you are really ninja. On a Linux distro it's easy - don't know about other OS'es.

The demo "Audio Input Device Demo" comes with LUFA and is ompatible with the atmega32u4 used on Leonardo. It can be set up to sample analog input and send it to the pc. From the documentation:


  On start-up the system will automatically enumerate and function as a USB microphone. By default, the demo will produce a square wave test tone when the board button is pressed. If USE_TEST_TONE is not defined in the project makefile, incoming audio from the ADC channel 1 will be sampled and sent to the host computer instead.