Everything in nature is doomed to fall prey to "dissolution," or extinction. Everything changes. Because dissolution affects the full range of nature – high to low, big to small – Wordsworth compares its action to notes on a musical scale. These notes make a kind of song, though a very sad one perhaps.
You can only hear this "song of change" if you have a peaceful heart. If you’re a criminal, a greedy person, or an egomaniac don’t even try. You’re not invited to the Mutability Party.
Truth never changes, but the outward forms of truth change. Appearances change, like the melting of a white frost or the falling of a sturdy tower that has grown weak with age. The tower is a symbol for the quiet and mysterious workings of change. It was once so powerful, but in the end it was brought down by the sound of a shout and the touch of Time’s hand.