To start, double check that all the files of the game are either in plain text English or Japanese characters.
If none of the files have messed up names, then download the trial and copy the Audio folder to your game directory, overwriting all files.
If even that fails, I'd recommend trying on another computer and posting the results of what happens on that one. Below are some other reasons this could happen, without even being related to the game itself.
Small and unlikely possibilities include the path length of the file is too long, the game folder may not have Read/Write permissions, your drivers are extremely outdated or not installed, Unicode settings for Japanese are not set up, or the Japanese language is not installed correctly on your computer.