Anyone interested in how Korean was pronounced roughly 800 years ago should look into the 계림유사 (gyerimyusa/ kyerimyusa), as it is constantly referenced in books on Korean etymology and phonology. The format of the book is such that roughly 350 Goryeo (ancient Korean) words are written out phonetically using Chinese characters.
The original meaning of words are shown with Chinese characters which are then followed by their pronunciation. The 曰 in the middle (not to be confused with 日) means "spoken / pronounced as." Here is an example of an entry from the book: 鬼曰幾心. This is read as: "Ghost (鬼) is pronounced as 幾心 (기신, kishim)," which is basically identical to the modern pronunciation 귀신. Naturally, there are also cases where the pronunciation indicated is no longer in use, as in 山曰每, where the word for mountain (山) is the archaic "뫼 (written as "매" with 每)."
While basic reading knowledge of Chinese characters will help, you can just as easily copy and paste or draw the characters into a Korean Hanja dictionary to see the approximate pronunciations.
The following blog lists all of the words and pronunciations with Hanja only:
For those who would like a version with modern Korean meanings and pronunciations, google "계림유사 txt" and it should be one of the first search results as a downloadable Word file.
Here is a Hanja dictionary for those who would like to look up the pronunciations of the Chinese characters: