To get the best karaoke effect, download those karaoke MTV that
has 2 channel, then use the old media player to play it, you can
swith between stereo(both channel) or one of the channel
only(either music or singing only).
if you are talking about mp3, i am not sure.
But donkey years ago when i used winamp(now i prefer sonique), i
tried one of the pulgin which can make the singing part of the mp3
song very soft, and the music is still normal. The effect was not
that good though, not close to karaoke feel.