Krazy Kat is a gondolier in Venice; that's in Italy, so he sings "Santa Lucia." As he and his fellow gondolieri travel through the canals, it turns out that everyone sings opera, even the cats and dogs.
Im not sure who actually sang the arias, choruses and other songs in this cartoon, but they're pretty good renditions, and the quality of the soundtrack was more than decent. I also thought the variety of gags in their rat-a-tat timing were pretty good, even though they had to spend about thirty seconds on that exotic dish, spaghetti.