In the third and final stanza of "To Autumn," the speaker describes the sounds of the autumn season. He describes first of all the "wailful choir of the small gnats." The word wailful suggests that the sounds made by the gnats are mournful and sad sounds. The phrase "wailful choir" also suggests that the sounds made by the gnats are loud and discordant.
Next the speaker describes the "loud bleat" of "full-grown lambs." The sounds made by these sheep, and also the aforementioned sounds made by the gnats, are both described as loud, and the overall impression seems to be that the sounds of the autumn season are not calm or peaceful but are instead disruptive and unpleasant. The word bleat also perhaps implies sadness or fear, as sheep often bleat to indicate irritation or danger.
The next sound described by the speaker is the "treble soft" sound of the "hedge-crickets sing[ing]." The word treble indicates a high-pitched sound, which adds to the overall impression that the sounds of the autumn season are discordant and unpleasant.
The final sounds described by the speaker, in the final two lines of the poem, are the "whistles" of the "red-breast" robins and the "twitter[s]" of the "gathering swallows." The word whistles again implies a high-pitched sound, and the word twitter suggests restlessness. Altogether the different sounds of the autumn season, described by the speaker in the final stanza of the poem, seem to be somewhat discordant, disruptive, and unpleasant. This is perhaps to reflect the connotations of death and decay that are associated with the autumn season.