This amazing performance comes from “Les Petits Chanteurs a la Croix de Bois,” a well-know boys choir in France. The song is called Rossini’s “Duetto buffo di due gatti,” aka The Meow Song.
From the Wikipedia page for the song:
The Duetto buffo di due gatti (“humorous duet for two cats”) is a popular performance piece for two sopranos which is often performed as a concertencore. The “lyrics” consist entirely of the repeated word “miau” (“meow”).
While the piece is typically attributed to Gioachino Rossini, it was not actually written by him, but is instead a compilation written in 1825 that draws principally on his 1816 opera, Otello. The compiler was likely the English composer Robert Lucas de Pearsall, who for this purpose used the pseudonym “G. Berthold”.