Soundwaves and radio waves have similar if not identical properties. An extreme example of phase cancellation occurs when two or more ( more makes the effect more pronouced ) signals from the same transmitter arrive at your receiver at slightly different times. For anyone who has ever listened to a distant AM station at night or a distant shorwave station, you will notice that when you listen to the station, the audio quality becomes slightkly distorted as your receiver picks up multiple signals from the same source. This is due to the phase differences that take place when two or more signals are mixed. It becomes a lot more pronounced though because not only are the signals being mixed, they are also arriving at different times as one or more signals travels over a much longer distance ie ..it is reflected off the ionosphere or possibly traveling around the world ( aka "long path" ) to get to your reciever, while the closest signal may be traveling almost directly to your receiver from the antenna.

Although I'm not suggesting that the distortion would approach what I describe above, it is present in a similar manner ( as Simon suggests ), and it will have some effect on the sound quality, depending on what effects are used and how much phasing is introduced.

AJ