Warren had a remarkably well-produced voice, with a naturally wide range, with secure high notes, and smooth, rich timbre throughout. He was most associated with Verdi, which he sang with a good deal of artistry and feel for the natural line, though he also excelled in Puccini (especially Scarpia) and verismo.
He first planned on a business career, and studied for a year at Columbia College, but in 1933 decided to quit that to pursue a singing career. He first studied at the Greenwich House Music School, and in 1935, auditioned at the Radio City Music Hall. He had hoped to become a lead singer, but Robert Weede was the reigning baritone there, and Warren was just offered a place in the choir. He sang there for the next three years, augmenting his income with the occasional radio program, wedding, or funeral, and studied with Sidney Dietch, and eventually made it to the Metropolitan Auditions of the Air in 1938. When the Radio City Music Hall refused his request for a few weeks off to prepare (he knew only a few arias and had never sung on the opera stage before), he quit, and threw himself into preparations on his own. Legend has it that at the audition, the conductor Wilfred Pelletier rushed backstage, convinced that they were playing a prank on him, and Warren was lip-synching to a Ruffo or De Luca recording.
He won, rather to his own surprise, not only the auditions but a stipend to study in Italy with, among others, Giuseppe de Luca and Riccardo Picozzi. There, he learned five complete roles in less than seven months, despite having seen just one complete opera in his life. He made his Met debut, which was also his staged opera debut, as Paolo in Verdi's Simon Boccanegra in January of 1939 (Tibbett sang Boccanegra), and soon became a favorite baritone at that house, singing all the major Verdi baritone roles. At first awkward on stage, he studied acting, and while never a great operatic actor, became more at ease on stage and put a good deal of thought into his interpretations. Like many opera stars of the time he was offered film contracts, and made his film debut in 1949 in When Irish Eyes are Smiling. He created the role of Ilo in Menotti's The Island God (which Menotti withdrew shortly after the premier). Like his successor, Robert Merrill, and to a lesser extent Sherrill Milnes, as well as his predecessor, Lawrence Tibbett, his artistic home was the Met, though he did perform in other countries, making his Teatro Colon debut as Rigoletto in 1942, appearing in Il Trovatore in Mexico City in 1948, and making his La Scala debut in 1953 as Iago. In 1958, he also made a tour of the Soviet Union. He had been suffering from high blood pressure, and died on stage at the Metropolitan during a 1960 performance of La Forza del Destino.
His Macbeth under Erich Leinsdorf, with Leonie Rysanek (BMG/RCA GD 84516), is excellent, and he and Bjoerling are both at their best on a recording, also with Leinsdorf, of Tosca (BMG/RCA GD 84514), though Milanov's vocal difficulties are something of an impediment to the complete success of the recording. ~ Ann Feeney