Completing the Century concludes the Metropolitan Opera Guild’s series of nine double-CD sets chronicling “100 Years of the American Musical Theater” by presenting 36 selections from Broadway cast recordings of 36 shows that opened between 1980 and 1994 on two discs with a running time of 141 minutes. The period is marked by the continuing dominance of composer/lyricist Stephen Sondheim, with songs from his scores for Merrily We Roll Along, Sunday in the Park with George, Into the Woods, Assassins, and Passion.