People have been asking for Japanese voices for a while already and it turns out that it's been possible to just manually enable them yourself all along. To do this, go to the game's directory and from there go to \Data\audio\Fmod_Build.
In that directory you'll see the files sac_bank06_voice_eng.fsb and sac_bank06_voice_jpn.fsb among a number of other .fsb files.
To enable the Japanese voices, you simply need to replace the .fsb file ending with "eng" with the one that ends with "jpn".
In other words: rename or delete sac_bank06_voice_eng.fsb, then copy sac_bank06_voice_jpn.fsb and rename that copy to sac_bank06_voice_eng.fsb.
I've played for several hours weeks after doing this and it works perfectly so far.
Note1: It's no problem if you do this while the game is running and you're sitting in the menu (you also don't need to restart the game afterwards).
Note2: if it doesn't work after replacing sac_bank06_voice_en.fsb, try deleting both sac_bank06_voice_el.fsb and sac_bank06_voice_jl.fsb, then copy sac_bank06_voice_jpn.fsb and rename it to sac_bank06_voice_el.fsb (sac_bank06_voice_jl.fsb should still be missing).