As copyright goes, you arent allowed to use anything from an artist until 75 years after his/her death. If you were to cover a classical piece then the person would have to be dead by about 1940. Unless the rights have been passed on to a buyer or family member.
Covers, although completely remade by yourself in copyright law is still illegal. You need documented allowance to re-create the piece of music, to do your own cover. So typically, anyone who makes a remix, without permission by the creator can be potentially sued.
The reason why it seems to be okay alot to make covers is that people to make alot of money from it. If it were to go public for money then you would most definetly have them on your tail. Samples are a no-no. From anything without permission. The misconception of it being under 30 seconds of a sample is a lie. It is a load of rubbish.
If you recreate something which is obviously made to be similar to a certain piece, (art or audio) then you can get sued for plagiarism. Something like a guitar riff, bass riff, specific sound, or theme copy then you are copying, hence breaking copyright act.
Copyright law is a hole different section to law. It's massive. And one of the modules I had to take for my Honours Degree. I recently had Animation Mentor pull copyright on me when I recreated a 3d character and animated him which was inspired by their character. LegalAction was implied to say that the character was copied. Even after I had linked the original inspiration back to their website. I made all the character, rig, textures, lighting everything myself, yet they still had the right to pull copyright on me. Which they did.
So, you may not make money, you may be inspired, but even being inspired can get you into trouble.