The idea conveyed through the comparison of humans to lyres is that like a lyre with strings that always make new sounds, humans respond uniquely to different experiences.
The second stanza from "Mutability" by Percy Shelley indicates that humans behave and feel in a different way in every situation they experience in life just as lyres play different tunes. In other words, the poet is stating that individuals have different responses to different life events. In that way, by using imagery, the poet is trying to remark the human condition of mutability. He highlights the idea that humans are always changing and there is nothing constant in their lives.