The sound device used in this excerpt from "How the Animals Lost Their Tails and Got Them Back Traveling from Philadelphia to Medicine Hat"? is alliteration.
Alliteration is a literary device which is a repetition of the consonants in a row, sound stressing syllable of a phrase.
In the above instance, "There were sixty-six representatives on the committee and they decided to call it the Committee of Sixty-Six" uses the sound device alliteration which emphasizes the bold phrases.