"were both singing "La Cucaracha""

Spanish for “The Cockroach,” this is the title of a Spanish narrative folk song that became hugely popular among Mexicans during early 20th-century Mexican Revolution. Adapted many times with alternate lyrics of a topically political or satirical, today it even constitutes the tune of car horns.

That Yossarian hears the two gunners singing it suggests how happy they are that he abandoned the bombing mission over Bologna.