The popular content was altered without permission by others (fan art/remixes) on sites like DeviantArt & Soundcloud.
I've used the remixed content they've created with their permission but they haven't asked permission to alter the original content.
Keeping my light hearted behaviour in mind, i haven't assumed i'd need to get so in depth. I was proven wrong. I made a mistake in being vague.
I understand what you're saying, no need reinforce, thanks for the advice.
If you buy a stolen cellphone or car, what do you expect will happen when the original owner approaches you about it? No matter what you paid or what contract you got from the thief, the car or phone is still property of the original owner. If he goes before court, you will be forced to return his property, and maybe also need to pay an additional fine for buying stolen goods.
As in ashamans answer above, you can TRY to sue the thief.... but while the thief will also be punished, most probably you will not get your money back.
Stolen IP is stolen IP. If you use other peoples IP without their permission, you are infringing, no matter if somebody else did the crime and you now just use the fruits of this crime for your own good. You are still infringing.
The creators of this images or music tracks most probably didn't get into trouble because their work didn't reach a big enough audience for the original IP holder to notice. Could you get away because your work will not get enough fans and nobody notices? Of course. Do you really want to be caught in a tough decision between failure to reach an audience or risk of being sued?
There is only one thing to do, the right thing: don't use unlicensed IP, be it music, or images, or stories. If you cannot live without it, try to get permission to use it, and pay whatever the original IP holder asks you to pay. If you cannot afford it, cannot get permission (because the IP holder doesn't want to sell his IP to you for example), or do not want to go through all this trouble, come up with your own stuff.
Its simple really. You can create a game about a ninja village, battling other ninja villages, and make the hero a red haired young boy. Nobody can patent or trademark broad story concept, nobody can claim the word "Ninja"....
If you call your hero Naruto, use the same village and clan names as in the anime/manga of the same name, or make your hero too similar (ninetails stuck in him, yaddayadda), you are infringing someones IP.
You can use much more than you think... "Ninetailed foxes" are actually chines / japanese folklore, feel free to use that as long as your story does not resemble Naruto too much. You can take a famous symbol and alter it a little bit, and you might be good to go... BUT: nobody can guarantee you you will get away with it if you walk that thin line between looking very similar and obvious plagiarism.
AFAIK apple has sued many companies over their lifetime for using a Logo with an apple. Most of the time they didn't had to sue them, they just told them "change it, or else...". In the end it doesn't really matter if they would or even could win this case. Almost no company could survivea legal war of attrition against the company with the biggest cash reservers in human history.
And yes, I brought up Naruto because this thread is like THE MIRROR IMAGE of a similar thread we had in these forum some weeks ago... which was about stolen IP from Naruto if I remember correctly.
TL;DR: Just come up with your own Stuff. Don't infringe on other peoples IP.