http://www.ni.com/example/31218/en/
There's help and examples available. Read your AFT documentation and search NI for tutorials on that toolkit. You'll also need to dump the DAQ Assistants and learn to write real DAQmx code since you need sample aligned waveforms for decent results. Also keep in mind that you will not be able to mimic real noise cancelling headphones very well. Without super fast ADCs and a real-time OS there will be too much phase difference to work for high frequency signals. There may be expensive DSP-Based products out there but most NC headphones are analog due to the phase issue.
It's a neat project idea though and you should be able to create a cool example of how the technology works. I can't help much since I don't know the AFT or NI-Elvis but keep us posted on your progress and we'll help where we can (Short of doing your homework for you of course :smileywink:).
You should do a search here for more info on noise cancelling too, I'm sure I remember similar requests in the past.