Kokuryuu Heika no Seiryaku Hanayome: Majo desu ga, Tasuketa Ryuu ni Yomeiri saseraresou desu

Olivia, a witch whose magic was once woven through song, lost her voice after saving a wounded black dragon as a child. Powerless and quiet, she now lives a secluded life with her elder sister—until fate calls once more. Summoned to the Dragon Kingdom as the savior of King Liberto, Olivia finds herself caught in a political tug-of-war. The cold, disinterested Dragon King has no intention of marrying, while his advisors push to find him a powerful bride. But despite his aloof nature, Liberto begins to visit Olivia, drawn to the gentle silence she carries.

(Source: MangaDex)