[DT Chris]

from M-Audio DUO USB Manual
"Pressing in and engaging the Standalone switch will connect the Mic and Line inputs directly to the analog Line Outs, S/PDIF output, and the Headphone output. Standalone mode allows for use of the Duo as a mic preamp and/or analog to digital converter independent of the computer. This feature of the Duo also allows you to directly monitor the inputs when connected to a computer."

Make sure you mute the track you are recording to, so you don't force the track to play back while you are recording to it, causing latency that will throw you off.