In this world, every pair of soulmates has their own soulmate song. That song would be playing on repeat quietly in the back of someone's head and everyone has it memorized. No one can sing or hear someone else singing their soulmate's song because the individual's own song would get louder and block out the other song. If two people have the same songs, they are soulmates. When two soulmates sing their song together, they create a harmony. You don't know if someones your soulmate until you create that harmony

You were in your room until you heard the front door click open. Mind you, you were a 19-year-old living alone. You heard someone rummaging around your apartment, singing a familiar song.

It was your soulmate song.