Here's what's said in the Folk Song Abecedary (James F. Leisy, Hawthorn Books, 1966):

There is a widely circulated theory that this song was based on the words "come by here" as Africans attempted to imitate these words spoken by missionaries. In any event, this fragmentary song became traditional in Africa, where it was "found" and brought to America, to become part of our tradition. As the world gets smaller and cultures mix, more of this tradition swapping is bound to take place.
The words are pronounced koom bah yah, and the song is usually sung very slowly and with dignity. Many people use the English words "come by here" instead. New verses are constantly being coined. A few examples are shown here. The chorus may be repeated after each verse.

Kum ba ya, my Lord, Kum ba ya.
Kum ba ya, my Lord, Kum ba ya.
Kum ba ya, my Lord, Kum ba ya.
Oh, Lord, Kum ba ya.

Someone's singing, Lord, etc....

Someone's praying...

Someone's hoping...

The only theory I found that was printed on actual paper was the one above, but I've also heard that the song was gullah, taken over to Africa by American missionaries, and then brought back as a so-called "authentic" African song. Whatever the case, it seems to be as good a case of misinterpretation and misunderstanding as you'll ever find in folk music. There are so many theories that it's doubtful that anyone could ever find out the truth.

Do I like the song? No comment.
-Joe Offer-