I have a soulmate AU idea! What if each sm couple has a song that they both relate to and understand. Once one of them finds that song then whenever they're soulmate is within hearing range they will accidentally start singing that song until the soulmate starts to sing along, but the catch is that they have to finish the song in perfect harmony in order to prove that they're soulmates. Anybody else will get off beat at one point or forget a lyric, but somehow that persons soulmate will know the entire song even if they've never heard it before and can stay in tune with that one person no matter how bad of a singer they are.