The Monsters of Music by Rebecca F. Kenney

Mel must share her creative magic or be driven mad by it. But finding her first protégé isn’t as easy for her as it is for most Lianhan Sídhe (muses of Celtic myth). Though the women of her race are naturally beautiful, she carries horrifying scars across one side of her face, inflicted by her mother’s obsessive boyfriend. And Mel isn’t only interested in pouring her creative energy into a man; she wants to use her musical genius herself, too. But the laws of the Lianhan Sídhe, and her own savage appearance, stand in the way of her ever singing onstage.
To relieve the painful pressure of her magic, Mel latches onto Kiyoji, a boy with a beautiful voice, and coaches him through a televised singing competition. But neither of them are prepared for the power of their connection, or for the new kind of magic that happens when the two of them sing together.