A Phantom Melody is an audio illusion. When a piece of music has high-speed arpeggios or other repeating patterns, an illusion takes place. The listener picks up the slight changes in notes, and mentally links the notes together to form a new melody not originally written. Take Sinding's Frühlingsrauschen (aka The Rustle of Spring). When listened to, the arpeggios mesh together into a separate melody.
Rustle of Spring - Full Speed
Rustle of Spring - Quarter Speed
The normal song has two melodies going on. One in the arpeggios.
When slowed down, the listener does not link together the notes.