Your mind is a wonderful thing, but it is forever making things up. Two dots and a line? That’s a face. Something small moving in the corner of your eye? That’s a bee coming to get you. A sudden cold draught? Must be a ghost. The same goes for sounds, watch the video, if you give an infinite number of monkeys an infinite number of pianos, sooner of later you will get the Bee Gees blaring out at you.
Apparently you are doing top-down processing, using previous information to make sense of the world around you, a sort of predictive text for brains. Every time a handful of notes turns into the Star Wars theme in your head (or the Superman theme if you get confused) this is what has happened. My heating system used to make a series of clicks that always reminder me of the Dance of the Sugarplum Fairy, turns out I wasn’t going mad, just had too much spare brainpower.
The New Scientist did an article on it so I don’t have to be coherent: