Mojibake, when Asian text becomes garbled, is always an issue for me when moving my music files from one computer to another. I'm probably the only one here but I have a Zune and accordingly use the Zune software. Sometimes the songs appear correctly with the CD art and the text carrying over properly; however, most of the time it's just a mess, and the process of fixing everything can take upwards of several hours. It seems to all be reliant on whether the person that uploaded the music did so correctly.

How can I change the music files so that this doesn't happen if the files are opened in a program that supports Japanese text? And is there any efficient way to do it? I'm aware I probably have to open up the file's information and alter it directly. Not really sure the best method though.

Hmm, I don't know anything about Zunes, but if you are on Windows you might want to make sure your "language for non-unicode programs" (google it + your windows version if you're not sure where it is) is set to Japanese.
Also, is your Zune set to Japanese (if possible)?

As for changing an mp3's metadata (title, artist, etc), most media players should have a way to do this. I've done it in Winamp & VLC, and it's usually under something like "Change File Info". If in doubt, Google something like "edit metadata [media player name]".

Yeah... windows is bad at this. Apple products do it beautifully... switch everything to apple is my suggestion... ;D
*boo hiss*