from M-Audio DUO USB Manual
"Audio from your computer is sent simultaneously to the Duo’s two
balanced/unbalanced Line Outputs on 1/4” TRS jacks, the coaxial S/PDIF
Output, and the Headphone Out."
Ok, it's not a problem with the audio interface. Perhaps you haven't told Adobe Audition which device will play back the audio.
Options->Device Order->Playback 
I have mine currently set to playback the wave files in Audition using my computer's soundcard (just for listening). It is set as the first device used in the list.
When I want to monitor my current recording and all tracks playing back, I need to use my Fast Track, so I would set this device to be 1st in the list instead.
Now all the audio playing from Audition should be routed to the Fast Track for monitoring.