We need to be careful here. A volta or turn is something that is associated with sonnets, which are poems that have fourteen lines and are written as one unit of verse with a set rhyme scheme. Normally when we refer to sonnets we think of Shakespeare and Petrarch, for they wrote the two acknowledged forms of sonnets in English literature. A volta is something that occurs towards the end of a poem and signifies a sudden shift or turn in what the speaker is saying as he or she may suddenly talk about how the topic of the sonnet relates to him or her.
This great poem is actually not a sonnet in the strict sense of the word, as it consists of a number of different stanzas, with each stanza written in a sonnet form. Therefore it is difficult for us to speak of it having a volta in the normal sense of the word as it applies to a sonnet. However, if we examine the poem, we could argue that the final section of the poem might represent the volta in the poem overall. This is signalled by its first line:
Make me thy lyre, even as the forest is;
The speaker expresses his purpose in this poem after describing the power and majesty of this wind in the rest of the stanzas. The poem changes after this volta through the way in which Shelley implores the West Wind to make him his instrument so that he might share and spread his ideas and philosophy over the earth to "quicken" their birth.