This mashup features a track that is well known to most gamers, the prelude from Final Fantasy VII.
It's not the original though but the orchestral version as performed by The Royal Stockholm Philharmonic Orchestra.
An Ordinary Fantasy
Music - Prelude (Final Fantasy Series) - Nobuo Uematsu & The Royal Stockholm Philharmonic Orchestra
Vocal - John Legend - Ordinary People
VDJ - An Ordinary Fantasy (Nobuo Uematsu vs John Legend) by VirtualDJ
other links to the mp3: