Make me thy lyre, even as the forest is: What if my leaves are falling like its own! (57-58)
When the speaker asks the wind to use him as its instrument (see our discussion of these lines and the "æolian harp" in the "Detailed Summary"), he implies that on his own he can’t communicate his thoughts to the world in the form of art – he needs the West Wind’s help.
The tumult of thy mighty harmonies
Will take from both a deep, autumnal tone, Sweet though in sadness. (59-61)
The speaker suggests that he’s not just a mouthpiece for the wind; he will create "music" similar to, but ultimately different from, the wind whistling in the trees.
Drive my dead thoughts over the universe Like withered leaves to quicken a new birth! (63-64)
The speaker believes that it’s important for him to communicate his ideas, not because they are great in themselves, but because they will establish an intellectual climate in which even better ideas can be generated.
And, by the incantation of this verse,
Scatter, as from an unextinguished hearth Ashes and sparks, my words among mankind! (65-67)
In a comparison rather different from the running metaphor of "dead leaves," the speaker compares his ideas to "ashes and sparks," suggesting that some of them are dead and others are capable of kindling new fires.
Be through my lips to unawakened Earth
The trumpet of a prophecy! (68-69)
The combination of the speaker’s mouth and the West Wind’s power will result in a "trumpet" that wakes the Earth – just as in Canto II, the metaphor here is the Last Judgment.