Kokuryuu Heika no Seiryaku Hanayome Majo desu ga, Tasuketa Ryuu ni Yomeiri Saserare Sou desu

Olivia is a witch who uses singing to perform magic. She lost her voice as a child, saving a wounded black dragon, and has since lived with her sister, a genius witch. One day, they are invited to go to the dragon kingdom, because the dragon that Olivia saved has become the king, Libert, and wants to thank his savior. There, they are faced with a dispute between the king, who does not want to get married, and his advisors, who want a strong witch as his bride. For some reason, Libert starts visiting Olivia frequently, and tells her she can't leave. What does he hide in his heart? And what's the secret behind Olivia's voice? Source: MU