In what century was the the style of voices popular that the Liebeslieder Walzer, Op. 52 was in?
Liebeslieder Walzer, Op. 52 (Brahms):  The waltzes are a collection of love songs in Ländler style for voices and piano four hands. Ländler: The Ländler (] ) is a folk dance in  time which was popular in Austria, south Germany, German Switzerland, and Slovenia at the end of the 18th century.
18th century