I was curious if this was possible, and I did some quick research. From what I gathered, it's just not practical. There are a multitude of mutually interfering sources and reflective surfaces in a noisy real world environment. You just can't plop a mic down - or an array of mics - and cancel the audio it receives. You have to put the mic and the cancelling audio as close to the ear as possible, and isolate the listener as much as you can. Which we already have, in noise cancelling headphones.