Complex sentence: By the time he was finally signed by the Metropolitan in 1940 he had appeared in most of the major international opera houses, including the Royal Opera, Covent Garden and the Teatro Colon in Buenos Airies.
Split into simpler sentences: By the time he was finally signed by the Metropolitan in 1940 he had appeared in most of the major international opera houses. In addition to those venues already named, he was heard at the Royal Opera, Covent Garden, and the Teatro Colon in Buenos Airies.