[I am not a lawyer. Below (and in the post above) is my layman understanding of copyright law]
Additional artwork were fan art (or traces) of famous animes that were also used with permission. I don't understand why they may remix original content with no issues, but when i used (already altered) content by themselves with permission i may
It's like when you are driving down the highway, and you see twenty cars going over the speed limit - some of them significantly more over the speed limit than you are - but you happen to be the one the police pull over and issue a ticket to.
Here we have to separate what people are doing (speeding, or making fan art) from what they have permission to do. I don't need permission to take a physical action, but if I don't have permission there can be repercussions if I'm the one getting caught. That is to say, the natural laws of physics don't prevent me from taking physical action, but the laws of men may attach legal ramifications to those actions.
If I wasn't aware of the driving laws in the USA, and I saw a huge number of people driving over the speed limit, it's fully understandable that I'd assume it's okay to do so. But that's why they make you learn the driving laws in the USA before getting a license.
If I wasn't aware of copyright laws (which apply to Europe and the USA and many other countries), and I saw thousands of people making fan art on the internet, I'd assume it was okay. The artists themselves are assuming it's okay. But that assumption doesn't actually make it okay according to our legal system.
If you're getting into business, or using property (characters, music, worlds) that belong to a business, you need to learn the laws that govern the creation, duplication, and distribution of created content.
I know this can sound really weird. "What the heck do I mean that fan art is 99.99% of the time in violation of copyright laws?!" (0.0005% of the time it's fair use*, and 0.0005% of the time they have permission**).
*Everyone claims fair use, but all the people claiming "fair use!" have no idea what that actually means.
**Percentages made up, but are within the right ballpark. Seriously, it's probably 99.999% that are in violation.
"How can someone "own" Mickey Mouse?! Well, I mean sure Disney created that character, so okay, maybe only they should be able to profit off it. But if I wanted to draw Mickey Mouse for free... why the heck shouldn't I? And then if I want to sell copies that artwork to a few friends... or take donations so I can continue to make free stuff... or put ads on my site to pay for hosting... or trade the artwork for some physical possession... or maybe just a digital possession... or maybe I should just take the latest Disney movie, and recreate my own version of it and release that for free... sure they're losing movie tickets because my Mickey Mouse movie is free and theirs isn't, but I'm not making any money... or maybe I can use Mickey Mouse, for free, to promote my favorite political candidate... or to promote my next non-Mickey Mouse commercial project... or just to earn me more Twitter followers... surely Disney shouldn't be able to stop me from making Mickey Mouse porn, right? I mean, sure it causes damage to their brand and intellectual property, but I am releasing it for free after all..."
If you create a character, or a game, or a movie, or music, or whatever... our laws grant you exclusive control over everything that is made with that character. You have the 'copyright' to that character. People can make similar characters, or use similar ideas, but you have control over your characters. That control lasts for only "X" years, afterward it becomes public property. Officially "X" is your entire lifetime plus 70 years* (which is far too long, but that's currently the law).
*Unofficially, it's basically infinite, because major corporations keep on getting the government to push the duration farther back, so no copyrights have been expiring.
Now let me quickly talk about derivative works. If I draw a picture of Mickey Mouse, it's a derivative work. It's my work, but it's using someone else's work. I own the parts that are mine (for example the background scenery, the pose I put him in, the lighting and camera angle, the composition, the outfit he's wearing, etc...), but they own the parts that are theirs (the character 'Mickey Mouse', which arguably is the most important part of the picture - people aren't fawning over the artwork because of my fantastic background scenery). My work is derived from their work. I own my part - my creation, but they own their part - their creation, which I was merely 'copying'. But they have exclusive control (for Walt Disney's lifetime plus 70 years) over the 'right' to 'copy' (duplicate, recreate, or otherwise use) their work (their character).
If I wanted to publicize my Mickey Mouse drawing (for example, by posting it for free on the internet) , I'd need Disney's permission (a license) to use their part of the work, and I'd need my own permission to use my part of the work. But if I made my own mouse-related character, that wasn't an obvious knock-off*, then I'd only need my own permission, because it's 100% mine.
*'moron in a hurry' applies to trademark law, but I think it's a good early-warning system for copyright infringement also.
Finally, back to your original question, if a music composer makes an original piece of music, he owns that music. If you want to use it in a game, you need a license from him (some recognizable form of permission). This can be as simple as an email saying, "Yep, you can use my music in your game".
The reason why licenses get more complicated than that, is to avoid misunderstanding (ironically ). You may think he means, "Yes, you can use my music in your game [without paying me]", and he may think he means, "Yes, you can use my my music in your game [in exchange for 50% of all the profits]". So people write out large more specific contracts that are seemingly nitpicky and pointless, but that actually spell out in detail what is actually being agreed to.
Time to throw you another potential curveball: You know how Disney owns some undefined portion of my fictional Mickey Mouse fan art? And how my art is 'derivative' and I need Disney's permission/license to use publicly their portion of my artwork?
Well, the actual real-life composer who's music you are actually wanting to use, who is giving you actual permission to use his work... his song contains Mickey Mouse in that song, and you also need Disney's permission to use his song in your game, because his song makes use of Disney's property.
Okay, slight exaggeration. His 100% original song doesn't use Disney's work, but it does use someone else's work other than his own, and you need that other person's permission. His song (likely) makes use of drum beats, flutes, or whatever else noises he used, that he then assembled into a musical melody. His composition is his work - but the sound "samples" (as they are called by composers) are owned by the people who recorded the sound. And if that recording is of a real person (say, a violinist, or a voice-actor), then they also own a piece of that work that needs to be licensed.
Okay, so now we're in a big muddy swamp of a problem here. Luckily, the solution is really simple.
The violinist gives permission (license) to the sound recorder to sub-license/re-license the sound to other people to make music with.
The sound-recorder gives permission (license) to your composer to re-license the sounds
When you license the composer's song, part of the license is (if it is written out in the license) a promise that he has permission to let you use whatever licenses are necessary for you to use the music in your game. This includes some several dozen different licenses from people you've never met and who he's never met. He's licensing not only his own work to you, but also (almost invisibly) re-licensing the components he used to make the music. He likely never met these people either - unless he's using live music, most likely he just downloaded some Creative Commons-licensed sound samples from some website like freesound.org. Those Creative Commons licenses (depending on the specific license) give him permission (license) to use that recorder's work in his work and re-license that work to you so you can re-license it to your users.
Eventually, your completed game is being licensed (and the music sub-licensed, and the sounds that make the music, sub-sub-sub-sub-licensed) to Valve so Valve's Steam service can copy your game (with your permission, since you have the exclusive 'right to copy' it) onto user's computers to play, and also your game is being licensed to the end users (EULA = End User License Agreement) so they can play your game, that contains the music, composed from sounds samples, that record a violinist.
You don't need to understand all of this immediately. Just pick up bits and pieces of it as you go, and you'll learn the important parts.
Really, all you need to understand (for the time being) is this: If you are going to release anything, free or not, make sure you have permission to use any components you use to make your game - and make sure the person who made the components has permission to use the sub-components he used.
He doesn't need custom permission - things like stock Creative Commons licenses work fine. If he's knowledgeable about his line of work (composing), he already knows all this. This is likely what he was trying to convey to you. If he's knowledgeable about his business, all you have to worry about is getting his permission for his composition, and he'll be taking care of the licenses for the sounds he used (that he likely already got before using them).
Here endeth my walls of text. Good luck on your development!