The vocals in those songs are clearly those of the original singers, James Hetfield and Brian Johnson, respectively. But how the hell did they get separated from the rest of the original bands’ instrumentation? I’m guessing the creator of these new versions did not have access to multi-track recordings of the original songs (from which he could have lifted just the vocal track), so how did he do this?
I did not listen to the examples you posted, but I’ve tried isolating vocals from songs myself.
First, I just search the internet to see if someone has done the work for me. There’s a lot of acapella’s available, sometimes artists release an acapella version themselves, or someone got their hands on the multitrack recordings, or they isolated the vocals themselves through unknown means. Results and quality vary - not every acapella is completely free of instrument sounds - but depending on what you’re going to do with it, the new musical accompaniment may mask whatever sounds or imperfections are present in your source material.
Second, there are some methods to separate the vocals yourself.
Sometimes you can do something with EQ-ing, but I’ve never had much success with that.
If the vocals are perfectly in the center of the mix, Audacity has a function that may be able to automatically isolate - or remove - vocals, but if there’s any kind of panning on the vocal, or reverb, this will not work very well.
I recently discovered a powerful piece of software called Melodyne, which analyses music based on harmonic content. I haven’t been able to spend much time on it yet, and it’s a rather time consuming process, but I did succeed in somewhat separating the vocals from Brigitte Bardot’s ‘Contact’ - the result is not great, but I was still pretty amazed by the software’s capabilities.
If anyone knows any other methods or tricks, I am very interested!
Well, assuming they are authorized samples, they may have been able to get the unmixed tracks from the original sessions.
Otherwise, I asked a similar question in this thread about the Mighty Mike mash up of Jump and Imagine. mikews99 pointed out you can extract tracks from Guitar Hero. I googled that, and there are several YouTube videos on the process, if you want more details.