I understood roughly what sou desu, sou des ka mean. But I'm having difficulty in understanding the word nan in sou nan des ka. If Sou des ka can literally be translate as "correct/it seem so is it?" How does adding nan "what" fit in this sentence? If anyone could explain would be much appreciated.
This なん is not 何; this is な + んです + か。
そう is treated like (but it's really not) a na-adjective. So following the construction here, it becomes そうなんですか。From the same website:
んです (ndesu) has the same meaning as です (desu), but is different in that it feels a bit more personal since it is used to explain something, give reasoning, or to emphasize something.
So corollarily, そうなんですか is like そうですか but it feels a bit more personal.
(Note: I believe this has already been answered somewhere in this site, but I'll answer it anyway for good measure.)