On my collages, i'm always trying (managing this is a pain) to credit the source i used, not because of copyright problems, but mostly to show everyone (and myself) where the original cut up came from. It gives much more information about creativity and resources to make other remixes.
And here's an music example i run onto this week:
I always loved Moby's Play album. Specially 'Run On' (see the music video). I knew it was a remix of some other song, but always forgot to go after the original. Remember, 1998/2000 it was MUCH harder (and fun) to chase this info.
So i came across this great slow motion video featuring Johnny Cash's version:
(see the Original Site)
After one google/wikipedia search i saw that both are based on a folk song called: God's Gonna Cut You Down.
Hopefully with semantic web this will all be done automatically.