I've also been looking for audiobooks in korean for quite a while. Unfortunately my last conclusion was similar to yours; audiobooks are not very popular or possibly closer to nonexistent. Podcasts seem to be very popular, but with those it's hard to find transcripts.
As for the Harry Potter series there is no official version either. And after having read through most of the first book, the translation doesn't seem very good either. Lots of spacing errors and names being spelled in multiple different ways. Regarding the accuracy of the language I cannot comment as I am still a beginner.
If you happen to find any audiobooks please let me know.
While waiting for my HP1 hardcover to arrive from GMarket I located a PDF, then later compared the two. My hardcover is the purple-cover revised edition (revised in 2013) and it has lots of differences from the PDF, missing paragraphs restored etc. I haven’t seen an actual paper pre-revised edition, so the differences I saw might be from mediocre OCR and spellchecking on the part of whoever made the PDF. But at any rate there was apparently enough demand to revise the book's translation 11 years after it first came out.
Meaning: I’m hoping this somehow means this is a good translation, since I’m already starting to read it! :)
My hardcover is essentially the version shown here (except that mine has "Harry Potter" written in English):
Edit: here's mine (trigger warning: ginormous GMarket URL):