This phenomenon is named after the lusty, half-bird, half-woman sirens of Homer's "The Odyssey." They beguile sailors with their seductive serenading, only to bring the poor individuals to their deaths. The siren effect uses singing to deceive (albeit unintentionally), having the potential to give average-looking persons a façade of sexiness, "luring" others in.
For instance, this is the reason (or one of the reasons) the Phantom in all musical productions of Andrew Lloyd Webber's "The Phantom of the Opera" always manages to be sexy.
Broadway Fan 2: "I completely agree, and the siren effect just adds to it. That voice is so seductive!"
Person 1: "Man, Sarah Brightman is so beautiful!"
Person 2: "I don't know, man. I think it's the siren effect."
Person 1: "Well, maybe it's both."
Student 1: "You know, I never found Matt that attractive before, but then I heard him sing. I think I'm in love!"
Student 2: "I know, right? It's a perfect example of the siren effect."