Our expectations, goals and knowledge plays a large role when we listen to sounds, for instance in speech and music. If we know what to listen for, we perceive things differently, than when we don't know what to listen for. Usually this is a blessing, since it makes living and listening easier in a known environment. But sometimes this can also cause trouble. Sound technicians and producers often encounter this problem when they mix the sound track for a record or a film.
Lets listen to a constructed musical example.
First listen to this melody. It's a well known American tune, but the tones are played in other octaves on the keyboard than usual, making it hard to recognize. Now , listen to this melody. This time the tones have been placed in the same octave relationship that we are used to. If you listen to the first example once more , it will probably be much easier to hear the melody 'standing out', even though the melody is played in different octaves on the keyboard.