Why do you suppose the listeners aboard the Nellie sigh?

***PART TWO***

At the end of the paragraph that begins, “The other shoe went….” Why do you suppose the listeners aboard the Nellie sigh? (Pgs. 46 - 47)

I think you are referring to,

“The other shoe went flying unto the devil-god of that river. I thought, ‘By Jove! it’s all over. We are too late; he has vanished..."

Marlow laments his newly discovered reason for his trip down the Congo; it was to see Kurtz. Marlow thinks they have missed him. I don't see the listeners sigh here though.

They sigh at the end of the paragraph.