Thankfully, it's been known that it's free, but I feel the same frustration. They're sound files that were already in the game before they were (mostly) replaced with English vocals. So why does it take so long to implement them, as if there wasn't enough disk space, anyway?
From what I heard as a rumor (so take this with a grain of salt), the JP voice were originally supposed to come packaged with the game on the disc, not as DLC since the Blu ray disc can hold the extra weight. They already got everything done, the licensing and negotiation with the JP side are also finished, so there's nothing holding them back.
However, the 360's disc can not hold any more information. The game on the 360 is already compressed enough as it is (and the game still have massive slow down), and adding this huge chunk of sound file in is not going to go well. So Koei was thinking of just having the dual voice option for PS3 only, but Microsoft does not want an inferior port on their system, so Koei was forced to equalize the playing ground by removing the voice track from both versions and release them as free DLC for both consoles. That way, both versions came out seemingly "equal" in contents, and it's purely depending on what console you have that decide which version get bought. It was a compromise
But we all know how that turns out to not be the case.
I'm not bashing the 360 here, this is just simply the truth. On hte 360 board, they seems to found a way to minimize this issue somewhat by lowering resolution, so good for them if they can play this smoothly.
Also, note that I said this was simply a rumor that I heard on another forum. Whether it's true or not is anybody's guess. Frankly, I find it weird that Microsoft even worry about such things for a game series that never sell well on their console anyway.