It appears I have an anomalous setup in my copy of Civ4:

In my Assets/Sounds, I have folders named "Resouces and Improvements" and "Resources and Improvements" (note the first word). I am wondering if anyone else has this identical setup, and which of these (if this is an incorrect setup) is correct.

I have Civilization IV/Assets/Sounds/Resouces and Improvements,

and I have Civilization IV/Warlords/Assets/Sounds/Resources and Improvements. The latter contains one file - OlivePress.wav

There is only one such folder in each /Assets/Sounds location.

One wonders whether the software can find the vanilla ones, since it looks like a spelling error.

I only have the folder without the r. There were definitely some sound issues in vanilla. I will just duplicate the folder and rename the copy with the correct spelling. An extra folder in there with the wrong name won't hurt anything. The Resouces and Improvements folder might actually be used, as long as whoever misspelled it was consistent. In your case I would try to make sure that both have the same files.

Weird, as both my Vanilla and Warlords installs have OlivePress.wav, and thus making my Vanilla install have the two folders. (Then again, it's also to be expected in a sense, considering that the same movie file is included in three different places in Mac C3C).