It starts at about 31'30" of that iPlayer recording, and it is the unaccountable music played at a ball in the Tuileries. I say unaccountable not because of any anachronism (although to quote Wikipedia
"Zola's 20 Rougon-Macquart novels are a panoramic account of the Second French Empire. They are the story of a family principally between the years 1851 and 1871" and – as the Cantique was published in 1866 – it was a close-run thing – the Tuileries was burnt down in 1871. [Perhaps the entertainers at the ball were singing from a proof copy... ]) But contemporaneity is not my biggest problem.
I just wonder whether sacred music with an organ accompaniment was a likely accompaniment to a glittering society ball in Second Empire Paris (for a start, how would they have got the organ through the doors of the Tuileries?...Unless it was already there. But then it would have been in what Wikipedia calls "The little-used northern wing of the palace, which contained the chapel, Galerie de la Paix, and the Salle de Spectacle [which] would be called into service only for performances, such as the Auber cantata performed the evening of Napoleon and Eugénie's civil wedding ceremony, 29 July 1853,"). Perhaps it is an ironic comment on the narrative, but I don't see how.... (unless Racine's "tout l'enfer" stood for what was going on at the ball; nah, much too subtle).
What made the music grate so painfully, though, was that the singers were English:
/repɒn su:r nu: lǝ fǝ dǝ tæ græsǝ pwi:sɒnte
kwǝ tu: lɒnfǝ fwi: ǝʊ sɒn dǝ tæ vwɑr/
.../ki: læ kɒndwi: æ lu:bli: dǝ teɪ lwɑr/ ...
(OK, I'm exaggerating a teensy bit; as with most choirs, only a few singers get it flamboyantly wrong, but those few stick out like a bear with a sore head).
What does this tell us about local colour? It had better be pretty damn good if it's to avoid breaking the spell of whatever it's supposed to be adding colour to.
PS A bit of an unfair clue, but quite pleasing (for me, at least):
"Bufo" for organic pest control? (8)
*This reference is explained here. When I say "it was the thirteen waters version, but they didn't sing those words" I'm not just being gratuitously contrary; it seems to me reasonable to suppose that before more egregious mispronunciations later in the piece they would have made the false liaison of très and haut.
Update: 2018.06.18.11:05 – Clarified footnote. And that answer is NEMATODE.