in order for a sound waves to interfere with one another they have to be hitting at exactly the same time and be hitting eachother at the same height, direction ect ect. if all these different variables happen to come together you would need an incredibly good acoustic environment to hear the difference. it does make an audible difference in a place like the vienna opera house ie when a cymbal player hits the cymbals together he moves one up and one down for this reason. but in a car or other poor acoustic environments it will not make a difference.
if two sources of waves are at different distances of a point, it is just matter of time for some wave to cancel another (even if its one in a billion wave) obviously the human ear would not notice this unless EXTREME circumstances (e.g. a hot spot for interference) but definitively adding more and more speakers increases the chances.