Cant once everything is took to one track all the frequencies bleed together. If you are just looking for the vocals I'd suggest searching for the accapella.
PEace
impossible, you will always have some noise in the background. it would take a hell of alot of EQing to drown out everthing except the part u need, and even then the quality would probably be too bad or distorted.
best thing to do is find the acapella as maddogg sed, and try and replay some of the instruments instead of directly sampling.