Ok, I am not sure how this happened. I have heard various accounts. But once and for all I want to clarify that the name of the big Malian (and Guinean) bass drums is dun dun. It is pronounced 'doon doon'. It is not djun djun. It is not pronounced june june.
The story I have heard about the origin of this calamity of mispronunciation is that it started with Babatunde Olatunji, who is Nigerian. Somehow he started spelling dun dun with the 'j' because that is how the French spell djembe. (Which IS a 'j' sound, of course.) His mistake has infiltrated drumming circles around the world and has resulted in generations of Americans mispronouncing the name of the drum. The irony is that Babatunde isn't from a country that even has these bass drums.
Another story I heard was that a West African drummer (could it have been Mamady?) kept calling the bass drums djun djuns and when he was asked about it by someone who knew that the drums are called dun duns in West Africa, he laughed and said that he thought that was the American pronunciation of the drum and didn't want to confuse his students.
So, ask a Malian, Guinean, Burkinabe, or Ivorian how to say the name of the drum and they will tell you: