Mary, it may not have been so much as what they did or didn't hear as it might have been that Missouri had no meaning whatsoever for them & as sea term & music normally go the words sometimes change so that they have meaning to those that sing them.
From the same area that we get the song 'Solid Fas' we get "Rayo Groun'" again a coruption of someting more meanful to the locals than "Rio Grand"?
(These are fro the British West Indies just in case any one' wondering)

This should be linked to the ongoing "Yaller Gal" thread

Barry