Barbie and the Diamond Castle (2008)

Liana and Alexa (Barbie and Teresa) are best friends who share everything, including their love of singing. One day while walking through the forest home from the village, the girls meet an old beggar who gives them a magical mirror. As they clean the mirror and sing, a musical apprentice muse named Melody appears in the mirror's surface, and tells the girls about the secret of the Diamond Castle.