A sound that is beautiful is the singing of a bird.
In the given lines, Shelley says that when we hear melodies, we enjoy them. However, later when we think about those melodiesand remember the lyrics and the music, they seem sweeter. This is because they evolve emotions in us as we hear the music in our heads and we appreciate its real beauty even more than we do when we are actually listening to those songs. That is why Shelley feels that when we hear melodies, they are sweet. However, when later we remember them and listen to them in our heads, they appear sweeter even though they are unheard.
