"Il dolce suono" ("The Sweet Sound") is the incipit of the recitativo of a scena ed aria taken from Act III scene 2, Lucia di Lammermoor by Gaetano Donizetti. It is also commonly known as the "mad scene" sung by the leading soprano, Lucia.
Lucia descends into madness, and on her wedding night, while the festivities are still being held in the Great Hall, she stabs her new husband, Arturo, in the bridal chamber. Disheveled, unaware of what she has done, she wanders in the Great Hall, recalling her meetings with Edgardo and imagining herself married to him.
Donizetti intended the aria to be accompanied by the eerie sound of the glass harmonica, though this instrument is often replaced in performance by a flute.
An arrangement of the aria was featured in the film The Fifth Element, sung by the alien character Diva Plavalaguna voiced by Inva Mula. Russian pop countertenor Vitas recorded a similar shortened version under the title "Lucia Di Lammermoor". While singing the part, Vitas changed the lyric "Edgardo" to "Esther" (hence the text "Esther! io ti son reso!").