Actually, I suspect it's not a Japanese problem, more a "lazy amateur dev facing a non-trivial technical problem".
You've got to remember that while some of the sounds (in the ruby makers' case) come from RTP, other do not.
You can assign a volume at which any given sound is played, but it's a bit tedious if you're lazy.
While there *is* a relatively straightforward way to normalize a set of mp3 files without reencoding, it's not the case for wav files.
Ogg files might have an option similar to mp3, but I'm not sure about it.
Reencoding should be avoided whenever possible, but doing this by file case requires keeping track of which file needs which volume value.
Also, in ruby makers, volume menu wasn't a part of the standard script set.
Still, I can't recall a title that was as bad at it as this one.
Anyway, any hints on that subboss ?