Your question is a little complicated but I think you are completely wrong!
You can't normally legally post anything anywhere or distribute it in any way or in any form unless one of three things apply:
1/ It is in the public domain - usually because it is old and copyright has lapsed.
2/ You own the copyright.
3/ You have permission of the copyright holder(s).
A song can't be "remixed"; a recording can. When this happens, someone is given access to the original multitrack recordings which are, literally, remixed: the relative volumes and eq of each track would be altered, effects would be added or taken away, parts would be removed and extra parts added. Copyright would then exist on the remix, the original recording (which was the basis of the remix) and on the song (the actual song as composed, NOT a recording of it).
Nowadays it seems like a lot of kids have no clue what the word "remix" means. Something can't be "pre-remixed" and, without access to the original multitrack recordings it isn't a "remix" at all.
Really, there is nothing to "look into". It's illegal to steal someone else's composition or recording.