Still heard in his soul the music Of wonderful melodies. (lines 31-32)
He wants to hear something from a poet with music in his soul. Romantic poets like Longfellow put a really high value on this kind of idea. They thought a poet was the kind of person who could hear music where others couldn't, who could see through the boring, painful details of the everyday. He could turn dull, normal life into beautiful music.
And lend to the rhyme of the poet The beauty of thy voice. (lines 39-40)
This is a cool image, isn't it? We love the idea of the reader's voice joining forces with the poet's rhymes. It makes the beauty of the poem into a team effort. The poem is only complete, only moving and real when the reader "lends" his or her voice to it. This is more of that stuff about the connection between the reader and the writer, the crucial link between the past and the present.