UA-1X: Using the UA-1X with GarageBand
The "USB Codex" is the Core Audio driver for the UA-1X. The faders in your recording software will act as volume controls. In GarageBand, make sure the USB codex driver is selected in GARAGEBAND>PREFERENCES>AUDIO/MIDI as the input device. I would recommend you hook up speakers to the output of your UA-1X and use the UA-1X as your Audio Output in both APPLE>SYSTEM PREFERENCES>SOUND and GARAGEBAND>PREFERENCES>AUDIO/MIDI. In this way, your UA-1X becomes you computer sound card, providing low latency.
You can then create a new track in GarageBand by going to
TRACK>NEW TRACK>BASIC TRACK>NO EFFECTS. You can set the Input Volume using the volume slider on the new No Effects track. You can now record your audio source by recording in this new No Effects track.