The antonyms for the word "out-of-key" are "in-key," "harmonic," and "melodious." These words are used to describe music that is pleasing to the ear and in tune. In contrast, "out-of-key" refers to music that is unpleasant to listen to because it does not follow the rules of harmony. When a musician plays or sings out-of-key, it disrupts the flow of the song and can be distracting for the listener. In-key music, on the other hand, creates a sense of harmony and balance, making it enjoyable to hear. Melodious music carries a natural flow and harmonious notes, playing in a soothing and delightful way.