its impossible mate, i wanted to do that 1 time also so i wrote a email to the magazine "Computer Music", w*w.computermusic.co.uk
And one of they engineers replied back saying that it isnt possble without the original file. The reason being that in a mp3 or wav file there are only 2 channels, left and right and no audio editer and make out what fequencys the vocals are on, because they are mixed with the music.
You can try playing around with the equalisers and some plugins but the result will sound rather amateur.
YOU NEED THE MULIT-TRACK RECORDING!!!!!!