The soulmates both have a song, when one of them sings it, the other can hear it. Both of the soulmates have different harmonies, but they fit together perfectly into this song. They find out they are soulmates when they sing it together.
(Bonus: soulmate songs are very popular in the media of the world, as few things fit together as perfectly, though, that's just considering the genres in which harmony is appreciated, there's still other popular songs, for other genres, ie, you are less likely to find screamo metal music from a soulmate song, so it really still depends on people's personal music preferences.)

~Sylva