The first ever “Muffled Voices” festival is set to give the Russian premieres of three American operas by composers Missy Mazzoli, Amy Beach, and Anne LeBaron. Mazzoli’s “Proving Up,” with a libretto by Royce Vavrek, Beach’s “Cabildo,” featuring a libretto by Nan Bagby Stephens, and  LeBaron and Thulani Davis’ “Blue Calls Set You Free” are among the ten chamber operas {…}