So I (with the help of others) have managed to get the entire Japanese VO working on my League of Legends using the files currently on the PBE, from what I've seen it works for all champions/skins and the announcer. Keeping this post short I'll just give you the steps on how to do it.

Re uploaded files since some people are getting errors - http://adf.ly/1UrTuy
or
http://adf.ly/1UrU7d

Should have put this at the start. DOESN'T WORK FOR MAC. Sorry
I will note that not 100% of the skins/champions have their VO. Some of the new champions/new skins do not have the VO. Its hard to test everything so sorry if that a VO you wanted doesn't work.
Reverting the changes. There is an option in Wooxy to uninstall the voice pack once you've made it that should revert the changes. If not back up the game incase of issues/wanting to revert. You can also download
http://adf.ly/1UrTpH
- This will let you choose any language to change (not Japanese yet)
FOR GARENA: Wooxy's listed file directory to the league client is D:/League of Legends/GameData/Apps/LoL. I successfully replaced the ingame English voiceover with the Japanese one. For the Champion Select voiceover, I went to D:\League of Legends\GameData\Apps\LoL\Air\assets\sounds\en_SG and replaced everything there.

Watch the video tutorial I made - http://adf.ly/1UrTYa
or..

Download the Japanese voice files from here - http://adf.ly/1UrTbf
ol_game_client_ja_jp.rar - (This is a Rar folder I created from the PBE files)

Download the program Wooxy
-http://adf.ly/1UrTRu
- *May need to be ran as Admin to work or
http://adf.ly/1UrTNa
Once you have Wooxy downloaded, we'll need to get all the files that we need. Unzip the folder that I provided and search it for the ".bnk" and ".wpk" files in the "managedfiles" folder and put them in a new folder.

Once we've got all the files into a new folder, using Wooxy we want to select the Skins button, then the Import skin button in the bottom left.

A prompt will pop up wanting you to add files. Select Add Folder and select your new folder with all the Japanese voice files. Once they have all been added, we'll be able to save the changes that we have done.

Once done, select the new file we have made in Wooxy and install it. We're done now!

Edit: 7. HOW TO GET CHAMPION SELECT IN JAPANESE :

Download ( http://adf.ly/1UrUQz ) (All the JP champ select sound files) or http://adf.ly/1UrUTU

Go to C:\Riot Games\League of Legends\RADS\projects\lol_air_client\releases\0.0.1.178\deploy\assets\sounds then select locale you use so en_US for me (You can copy the folder as a back up, then just replace the files in the folder)

Paste the .rar from step 1 and extract it, done

Thanks to /u/ImSoSte4my for finding out how to get the champion select working! Thanks to /u/imageeam for the Garena assistance.

Edit - As requested : VIDEO TUTORIAL - https://www.youtube.com/watch?v=M6ar2YmnOk0 *Sorry if its a really bad video, just made it quickly. Hope it makes it clear on what to do.

*Multiple edits for better wording/adding links/re-formatting