The soundtrack seems to feature two works from the "Virgil Fox plays the John Wanamaker Grand Court Organ Philadelphia" LP, released on Command Classics in 1964 (and available today from the Friends of the Wanamaker Organ, the Organ Historical Society and other pipe-organ organizations and various commercial sites). The pieces are "Fanfare from Parsifal" by Wagner and Fox's arrangement of Bach's "Come Sweet Death." The Wanamaker Organ is the largest playing pipe organ in the world and is noted for the incredible richness of its tone. For the short film, Lucas seems to have slowed down the playing of the pieces by about a half or a fourth, giving the eerie effect.
Review of Electronic Labyrinth THX 1138 4EB
