According to Audsley, Irwin and Sumner, these names have been used for a 16' or 8' stopped flute of soft tone. Irwin claims that it is sometimes open; Audsley maintains that it is made of wood. Williams notes that the names Sordun and Bourdon were sometimes confused. There is also a reed stop that goes by the name Sordun.
Of the dozen examples of Sordun in Oriris, none are known to be flute stops, and nearly all are known to be reeds. No examples of Sordon, Sourdine or Sordini are known. Contributions welcome.