Sound waves can be refracted through the atmosphere in different ways depending on the temperature and wind structure. Sometimes the refraction can lead to a dispersal of the sound, other times it can be brought to what you might term a 'focus'. I have more knowledge of this refraction as applying to sources of sound on the ground and how it can sometimes result in some areas receiving a louder sound and some a weaker sound, but no doubt the same physics apply to sound sources in the air.
Bear in mind that for an aircraft at an altitude around 35,000feet, the sound that you are hearing at the surface was created 20 to 30 seconds ago, not at the current position of the aircraft.