I used to think they sounded the same, then I actually bought Mayu and can tell how different she is by listening to her raw. And you're right, Mayu's voice is higher and raspier.
[Mayu, with a half-crazed expression, confronting Ia] Mayu: I love you, Ia! You and me, together forever! It was meant to be! Ia: Yeah... I don't think so. [Ia pulls out a list of names and shoves it into Mayu's hands] Mayu: These are... Ia: My lovers. [Ia turns and walks away with her nose in the air. Quick montage of Mayu talking to Meiko, Kaito, Miku, Rin, Len, Gakupo, Luka, Gumi, Kiyoteru, Yuki, Miki, Lily, VY1, Gachapoid, Iroha, Piko, VY2, Mew, Rion, Cul, and Yukari at various locations. Mayu confronts Ia again.] Mayu: I'll never accept it! I'm the only one for you! I won't let ANYONE else have you! Ia: Oh, such pretty words, _hypocrite._ Recognize THESE? [Ia pulls out a list of names and shoves it into Mayu's hands] Mayu: What... who... Ia: All those _other_ Vocaloids you're "the only one for". [Ia turns and walks away shaking her head. Quick montage of Mayu talking to Yukari, Cul, Rion, Mew, VY2, Piko, Iroha, Gachapoid, VY1, Lily, Miki, Yuki, Kiyoteru, Gumi, Luka, Gakupo, Len, Rin, Miku, Kaito, and Meiko at various locations. Mayu confronts Ia again.] Mayu: Okay, I'm a yandere, so I have to do _something_ completely irrational. Any ideas? Ia: How about Magnet? That one's super popular _and_ makes no sense whatsoever. Mayu: Fine by me. [They start to walk away but are confronted by an upset Sweet Ann, Prima, Sonika, Big Al, Tonio, SeeU, Oliver, Bruno, and Clara.] Oliver: What are _we,_ chopped liver?
