How this guy changed the voice of the music ?

I'm positive that the creator just swapped the audio during the editing process (before uploading it to YouTube). The audio most likely isn't in game.

 

It would be impossible to add this feature. It would lessen original songs. You would only have around 8 original songs and the others are covers. Let's say those 8 are sung by Miku, then there's Rin, Len, Luka, Meiko and Kaito.
8 multiplied by 5 is 40, which is almost the amount of songs in F.
Also, Project DIVA actually added the popularity of covers because of the module-changing thing while not changing the voice.

EDIT:

To be fair, SonicJeanCake still hasn't revealed his secret on how he made the Luka version of Miracle Paint, or anything that includes Custom DLC songs, to be honest.

 

Nah, it's quite possible, but you're looking at it the wrong way.

There are a number of games coming out that are using Vocaloid software to dynamically generate vocal content in-game. (The only ones I can remember off the top of my head are Project 575 (one game on iOS, one on Vita) and the new Band Brothers/Jam with the Band, but I know there are others.) What you do is just generate the vocals straight out of Vocaloid and then all you have to do (theoretically) is switch up the voice bank when you change characters. The problem is that most songs won't be tuned per Vocaloid, so they'll probably sound awful, but it's definitely possible. It just has to be built into the engine.

IIRC, Sega did Project 575, so I wouldn't be too surprised if we started seeing it in the Project DIVA games soon.

https://itunes.apple.com/jp/app/uta-yon ... 04218?mt=8

 

Which is why it probably happen. Each Vocaloid has individual tendencies due to the quirks of each voice bank. Peace Walker used the Vocaloid engine to allow players to make their Metal Gear say what they wanted (I believe that's how it worked, anyway), but, at best, it is simply a gimmick. Possible, perhaps, but not feasible.

 

Well it can't be too hard to get multiple vocals in. The Project Mirai 2 trailer just showed 5 songs that do it on a 3DS game.

 

Yeah, I suppose if that is the quality level you are looking for. I saw the trailer just a few minutes ago and wasn't ENTIRELY impressed (call my picky, if you like).

If that IS indeed to your liking, though, then it definitely looks like a possibility in the future. I just wouldn't expect that from the work of, say, Mitchie-M, seeing as his meticulous edits would likely require a total rework for each vocal.