Answer: Ephraim
In the song "Spring, Spring, Spring" several of the brothers sang solos to their brides-to-be. According to the CD booklet released with the Original Motion Picture Sound Track, the beautiful voice heard here is not that of actor Jacques d'Amboise, who played Ephraim, but one of several singers whose voices were dubbed in to substitute for some of the actors. It is not clear from this source which vocalist sang Ephraim's line, but non-acting vocalists credited in this song include M. Spergel, C. Parlato, Robert Wacker, Alan Davies, Gene Lanham, and Bill Lee.