Why might Chef Louis say "Au revoir" to his "poisson" instead of "Adieu?"
"Au revoir" is used to mean goodbye, but meaning they'll see them again, as opposed to "Adieu" which has a finality to it. He'll see the fish again soon when he takes it out of the boiling water to serve it!
What song is being played in the background as Chef Louis is chasing his poisson?
The Can-Can is playing. This a a classic French dance song commonly used in places such as "Le Moulin Rouge" at the turn of the century around 1900. This period in France was known as "La Belle Epoque."