As for the legality: It depends. It really depends under what license the person posted their xrni (look for “Creative Commons” and/or the abbreviation “CC <something>”). And if they didn’t specify anything it’s just normal copyrighted by default. Unless this forum itself some kind of License Agreement that “everything you post here falls under CC something”–but I’m 99.9999% sure that it most definitely does NOT, because hardly nobody (smart) would post their songs here.
So what does that mean? It’s copyrighted, but can you remix it?
I’m not sure, it’s kind of vague and it really depends who you are, who (legally) represents you, who (legally) represents the original artist, etc. Also depends on where you are and where the original artist is (what jurisdiction). For instance, if the artist is in the US and they are just some random person without a lot of money or fame, you can (afaik) do pretty much whatever since if they want a side of justice with their default copyright they have to pay up, a LOT, because such is the accepted way it goes there. If you live in the EU and you have any kind of legal insurance that covers international stuff, you can legally fuck them sideways and that’s legal. So very legal, it might make you pause and wonder if it’s right. … get where I’m going with this yet?
Similarly, people remix stuff by famous/rich artists all the time and seem to get get away with it just fine. The worst is their stuff gets taken down from wherever they were hosting it. It’s different if you try to earn money with those remixes, but who does that (again, live on a different continent and the rules get even more vague).
Now for the more important question, because we’re people not machines:
Is it rude?
First, everybody gets to have their own opinion about this. So I can only speak for myself and what I’d do: I’d do my very best to contact the original author and ASK THEM. They’ll probably LOVE the fact that someone found their weekend-project inspiring enough to build upon. I know that I would (so check my soundcloud!). Best is if you do this before you start remixing, but I can understand just randomly playing around with other people’s stuff and see what clicks and inspires, you know? But IMHO before you release (e.g. post to Soundcloud), you should let them know first. Or send them a link to a private track on soundcloud or whatnot, you get the idea.
But remember that this is art, and these people are artists. Art is personal. So maybe they just feel like, this is my track and it’s my musical expression of something very ineffable and you can’t remix it because they’re not sure if you’ll trample all over its ineffability and this will diminish the very personal value this precious snowflake has. In that case, I kindly suggest you back away, slowly. Because legal problems might just be the start of it
So it’s legally nice and personally nice to ask the artist. See it as a collaboration. On the other hand, since really nobody really makes any money with this, the foremost important thing is that you LET THE ARTIST KNOW because we need this, we need to know someone thought our track is cool enough to remix, we need to know people we don’t know actually found and listened to our tracks, we need to know what you think, we are so fucking vain please tell us we’re pretty please