If you remix something or do a cover, you own the copywrite to THAT piece of work, but not exclusive to any melodic and/or rythms used.
Pretty much, it's Newgrounds, about 80% of the stuff on this site is illegal in some ways, but no one cares, it's all about personal intertainment, no one's going and selling thier stuff on itunes or trackitdown, so just relax, the screning is quite slack.
Just make music, upload it if you contributed and didn't just totally rip it off "Like my rework of the main theme of Chrono Trigger where I took the midi and reworked all of the sounds as a test".
And hey, if they ban you from uploading , boo hoo, it's only Newgrounds, if you were really serious about it than you wouldn't be uploading rip offs or covers or remixes :)
It's all just fun and games here.
(approves any random banter or un edjumacated flamming now)