Here’s the scenario. You’re in your car, on the subway, in the supermarket. A song you know comes on. Maybe it’s the Everly Brothers’ “Cathy’s Clown” or the Beatles’ “Love Me Do” or Extreme’s “More Than Words.”
You’re singing along, at full volume, not just the basic melody, but a harmony vocal either explicitly in the song or in your head. Now consider that you’re probably singing a song recorded at least a few decades ago.