According to Wikipedia:
The line "singing Italian songs" is followed by "Eh Cumpari" and then Italian-sounding nonsense words, in the studio version of the song, rendered in the printed lyrics as "?". Piano, guitar, and vocal sheet music arrangements have often read "improvised Italian lyrics" in parentheses after this line. However, in a film of Chicago performing "Saturday in the Park," at the Arie Crown Theater in Chicago, in 1973, Robert Lamm clearly sings, "Eh Cumpari, ci vo sunari," the first line of a song known as "Eh, Cumpari!", which was made famous by Julius La Rosa in 1953.
So now you know.