And without instruments and microphones created by others, there would be no sound to create in the first place. Everything is derivative of something else on some level. And creativity can’t be judged on a black/white basis – there is a spectrum spanning millions of shades of grey (album). Anytime an artist adds something — even if it’s to something that already exists — that artist has created something different. There is creativity involved. Sorry that it doesn’t measure up to your impossible criterion, but yes, remixes, mashups, etc are indeed creative — and art.