Now that the game is fully updated to the Japanese version, you can now extract out the files required. (Note that this is actually similar to the andriod version and people who modded it in the android version should know how to do this)
Navigate to your steam folder, which should be in the C:Program files by default. Go to SteamLibrarySteamAppscommonShadowverseShadowverse_DataStreamingAssets
You will see a folder named "v". It should contain the Japanese voice files if you did Step 1 correctly.
Copy it out into another folder that is not in the Steam files. This is to store the files you need at another place as the ones in the steam files will be overwritten when you change it back into English. (Note: The "m" folder contains the movie file of the story mode, the "a" folder contains japanese image/animation files)
You now have the Japanese audio files. (Some has noted that you might want to start the game before copying the files, I have done it without starting the game though)