Use the adjective dulcet to describe a sound that is soothing and soft, like "the dulcet harmonies in a 70s pop song" or "the dulcet tones of a harp."
The word dulcet worked its way into English by way of the French word doucet, which is related to the word doux, meaning “sweet.” Originally applied to anything sweet or pleasing, such as something that tastes sweet or a sweet glance from a stranger, nowadays the word is most often used to describe sounds that are gentle and melodious — sweet sounds.
