Complex sentence: The practice has existed since before the advent of Broadway cast albums in 1943, when the songs from Rodgers and Hammerstein's "Oklahoma!", performed by the show's cast, were released on a multi-record 78-RPM album by American Decca.
Split into simpler sentences: The practice has existed since before the advent of Broadway cast albums in 1943. That year the songs from Rodgers and Hammerstein's "Oklahoma!", performed by the show's cast, were released on a multi-record 78-RPM album by American Decca.