IS ALL COPYRIGHTED MUSIC CONSIDERED PLAGIARISM UNDER THE RULES?

Google Dictionary
pla·gia·rism/ˈplājəˌriz(ə)m/
noun
noun: plagiarism; plural noun: plagiarisms

the practice of taking someone else's work or ideas and passing them off as one's own.
"there were accusations of plagiarism"

The key there is "passing them off as one's own". That's what we don't want you to do.

First: wherever you get your materials, it is insisted upon that you credit the creator.

So, generally, the site doesn't want you taking from fellow creators in the community without their direct permission. However, (addressing what the rule says here: "As far as graphics go, there is some leeway") that was rampant for years, so there are a lot of graphics floating around that nobody knows where they come from. They're just sitting on old lists all over the internet with no source and so far, it's proven extremely difficult to figure out who made them. So, if you're using any of those graphics, this site, at least, wants you to do your level best to make a good faith effort to credit them. If you can't, it's a generally a good practice to say in your credits, on your gamepage, somewhere visible, that you found these graphics and you can't figure out who made them.

So basically, if you use something that you didn't create yourself, get permission from the creator, pay for assets, or at the very least, or if you can't find a source, make it clear that you don't have a source for some asset you're using. If the creator's around, and won't give you permission, don't use it. And in general, it's not hard to get materials that you can use free and clear. There are creators who make whole sets for you to use and you can just go to their sites and download them, and they only ask that you credit them in your game. White Screw, Milano-Cat, First Seed Materials, etc. Some of these do specify that you can only use their content non-commercially (White Screw and Milano-Cat both ask that you seek permission for commercial use.).

If you're stealing from main-stream games, like Final Fantasy or something, that's frowned upon, but nobody's going to stop you from doing it unless somebody sends a Cease and Desist (which, that happens on occasion.).

Directly addressing music:

Obviously, if it's something one of us made, the the same as above is going to apply. But music is something that seems to be more tightly controlled. Being a music composer, I can tell you that I can't just dump my music on whatever lists. So, I think that you're just generally going to have a harder time finding derelict music assets because the sites that you can put music on are just, frankly, better policed, and music assets aren't something you're just going to find randomly floating around nearly as commonly.

If you use music from someone else, you can buy music packs from rpgmakerweb, from itch.io, I've got a pack in the store on this site (it's hosted on itch.io), you can find music that people just put out free to use with credit, such as Kevin MacLeod at incompetech, but in general, don't rip that stuff. Again, if you rip it from a mainstream game like Final Fantasy or Fire Emblem, it's frowned on, but nobody here is going to stop you, but music's a little more hazardous than graphics.

Copyright bots look for music. Music is the dangerous one to steal because everyone is looking for streams and music that SOUNDS like their content. And sites are really stupid about music. I transcribe a lot of classical music from the 1800s and 1700s to use in games or wherever. It's public domain! It's mine to use! But copyright bots will almost always flag them as belonging to someone else. I'm pretty sure Andre Previn would not appreciate his 1976 composition of Swan Lake being compared to my Swan Lake made in MuseScore using a public soundfont! I'd be insulted if I were him! (Ask me how I know Andre Previn conducted a performance of Swan Lake in 1976, or how I even know who he is! I'll bet you can guess!)

So, with music, if you use something copyrighted, anything! Add a warning. There is some streamer or let's player somewhere that's going to play your game, no matter how little it is, and wouldn't it be nice to let them know that if they don't turn down their sound, their channel's gonna get the hell struck out of it?

it's not really plagiarizing though I guess I would credit the music
I did that in a real "old" game of mine, I credited Nobuo Uematsu. :) Plagiarizing is when you take someone's work and say I did it. No!!! NOOOOOOOOOOOOOOOOO!!!!!!!
And that is not nice!!
Credit, credit, credit. Maybe at the end of your game or in the beginning.