From Muppet Wiki

Marilyn Horne (b. 1934) is an American opera star and noted mezzo soprano whose early career highlights included dubbing Dorothy Dandridge for the movie Carmen Jones. Horne has appeared on Sesame Street, singing "Sing After Me" with Ruthie and performing "C Is for Cookie" as Cleopatra. (EKA: Episode 3189)

Horne was the subject of a quick sight gag in The Muppet Show Episode 409, when "Marilyn Horn" appears: a trumpet in a blonde wig.