I'm trying right now to put the jap voices into the (U) dump with dslazy. I'll edit this post when I get it.
Ok, I think I've got it, with DSlazy. Swap the stream folder from the (J) rom to the (U) rom. Only tested this as far as the beginning. The stream folder in inside
That's if you used dslazy to unpack/repack the rom.
Seems like the (J) is slightly different, while playing I notice that in some areas where one person is talking you hear the voice of another person.