Before I tell about a podcast lecture that touches upon the Kingdom of Ghana (not the same as contemporary Ghana) a few words about UCSD. This university delivers some great content that is easily accessible on line. The current courses available contain among others two series about Medieval History (MMW3). My advice to you is to download all lectures as soon as you can, for further use. I found out that UCSD rather quickly tends to archive those courses and then empties the feed. This happened for example to the course CATS 2 about the interaction of science, religion, culture and society. The course was given earlier this year and has been removed, before I could finish it.
Now to Ghana. This medieval kingdom is touched upon in the UCSD series MMW 3, more specifically in the series by Professor Herbst in lecture 15 (May 20th, 2008). Mostly, Africa has been tribal, but a kingdom could come into existence because of trade. Trade became possible as soon as the Arabs and Berbers figured out, how to cross the Sahara with camels. With the camels they brought in salt from the salt pans in the Sahara and traded it, most importantly for gold. Ghana was a kingdom situated roughly where today are Mauretania and Mali. The gold was not from their soil, they got it from peoples further south. With trade, also Islam reached this part of the world. Soon the elite of the Kingdom of Ghana became Muslims.
I was wondering whether also in earlier times gold made it from West-Africa to the rest of the world. Maybe through Egypt or Ethiopia. There are so many questions and so few podcasts with answers. Maybe there are very few sources, but I wouldn't be surprised if there was also little research. Africa still lies largely undiscovered.
More MMW 3:
World history guided by the religions,
World history outside the European box,
Making of the Modern World - UCSD,
UC San Diego's podcast courses.