During the Kingdom Hearts Orchestra - World Tour - in Japan, the Japanese voice actress of Kairi and Xion, Risa Uchida, came onstage and performed lines. The Japanese voice actress for Aqua, Megumi Toyoguchi, later came onstage and performed some lines as well. These lines are canon to the story of Kingdom Hearts and explain some back story for Kingdom Hearts III.
You can view the translated Kairi lines thanks to @momo_krys on Twitter below:
I didn't notify you but you might've already heard from Yen Sid-sama.
I've come to train to become a Keyblade wielder.
Until now, I've only been waiting for you guys to come back from your journey.
Maybe with this I can at least help out a little.
The magician Merlin-sama can use magic that surpasses time.
Here, we can forget about the flow of time.
Isn't it great?
Lea's training with me.
He apologized to me over and over again.
But he'd keep on apologizing...
At first I was a bit scared but
As we trained together we started talking to each other.
I found out Lea also has a best friend he wants to save.
I felt like, he's not a person that I can bring myself to hate...
Sometimes..... He'd stare at my face
So I'd ask him, what's wrong?
"I don't know, but I feel like it's something I must recall", He'd say...
Your journey is a journey that will help many people
and the people you'll meet from now on
I have a feeling these people you meet will also need your help
The journey might be hard but
Please stay as the cheerful and bright Sora I know
Because your smile, will save many hearts.....
You can view the translated Xion lines thanks to @momo_krys on Twitter below:
On the day you first called my name
It made me feel like I had a heart
If I hadn't met the two of you
I wonder how my "everyday" would've been.
Day by day, repeating the days which were the same as every other,
The red, setting sun,
The taste of ice cream.
Those days when nothing happened were the most precious to me
All of it is my treasure.
Thank you for calling out my name.
Thank you for inviting me to the clocktower.
Thank you for picking up shells for me.
Thank you for coming to bring me back.
Thank you for letting me be your friend.
I wish... those same old days could have continued on longer....
We wanted to go to the sea....
@momo_krys hasn't translated Aqua's lines, but he says they were pretty much something like "Ven, Terra, become the light that will guide me through the darkness!" He says that it was essentially just her saying that she wanted Ventus and Terra to guide her in her darkness and to become the light that shines her path.
If you didn't know, last year during the Kingdom Hearts Concert - First Breath - in Japan, a similar situation to this happened. The Japanese voice actor of Terra and the Japanese voice actress of Naminé came onstage and performed some canon lines for Terra and Naminé. You can view those lines here.
UPDATE 1: View images of Kairi and Xion's voice actress, Risa Uchida, and Aqua's Japanese voice actress, Megumi Toyoguchi, performing their lines thanks to Game Watch below:
UPDATE 2: For all the other orchestras around the world, they played recordings of these voices with English subtitles. The recording of Kairi's voice played while Dearly Beloved played, the recording of Xion's voice played while Vector to the Heavens played, and the recording of Aqua's voice played while Wave of Darkness played.
UPDATE 3: After performing at the final Kingdom Hearts Orchestra - World Tour - in Osaka, Japan, Risa Uchida expressed her feeling of gratitude on Twitter and provided a couple photos of herself at the event.