An introduction to computer audio
There was a lobby going on at the Adroid Bug Tracker to include USB Audio support in Android. With success as Android 4.1 (Jelly Bean, July 2012) supports USB audio.
USB audio output support allows hardware vendors to build hardware such as audio docks that interface with Android devices. This functionality is also exposed with the Android Open Accessory Development Kit (ADK) to give all developers the chance to create their own hardware.
The Samsung Galaxy S III is one of the first, maybe the very first to work with conventional USB DACs.
Conventional? Yes, Google proposes to reverse the roles. The DAC becomes the host and the phone the USB device.