It sounds like whatever the creature hearing it desires...that will make them move toward the sound.
Bigsword the Fighter hears a very princessy-sounding woman crying out in distress.
Bowyn Darro the Elf Ranger hears a melodious elfin folk-song and wasn't aware any of his kin lived in these woods.
Macen Shieldalot the Cleric hears the rhythmic chanting of his cloistered brothers or someone shouting known sermons from his religious texts.
Hairfoot Thiefybits the Halfling Rogue hears the barking of his family dog that ran away when he was 10.
Greybeard the Mage hears the otherworldly soaring harmonies of the Sylphic Symphony who, it is said, are bound by cosmic law to bestow unearthly magical power upon any who view their performance.
Siren songs work the same way.
Satyr pipe music works somewhat differently, with everyone hearing the same music/notes with more of a hypnotic effect.