déchanter

Almost always used to mean losing one's illusions or optimism, not literally "un-singing" — il va vite déchanter warns that someone's high hopes are about to be dashed by reality. Intransitive, and typically used with adverbs like vite or rapidement.

• Il pensait gagner facilement, mais il a vite déchanté. — He thought he'd win easily, but he was soon brought back to earth.
• Les nouveaux employés déchantent souvent après le premier mois. — New employees often lose their initial enthusiasm after the first month.
• Elle risque de déchanter si les ventes ne décollent pas. — She may well be disappointed if sales don't take off.

The literal musical sense (to stop singing) has essentially disappeared from modern usage — using déchanter to talk about actual singing, rather than dashed optimism, will sound like a mistranslation.