What constitutes the music of autumn, according to the poet in "To Autumn"?

In John Keats's poem "To Autumn," the speaker declares that autumn has its own unique songs. According to the speaker, the songs of autumn include the sounds of gnats that resemble a mournful choir, the songs of river sallows, the loud and strong bleats of nearly grown lambs from the hills, the songs of hedge-crickets, the whistles of red-breasted birds, and the songs of swallows singing in the autumn sky. The speaker notes that autumn, though heading into the tough and trying months of winter, has its own gorgeous and unique songs. Rather than songs of new life, the songs of autumn are more solemn and strong as animals that were newly born in the previous spring are now growing into adulthood.