Quoting Ismael Valladolid Torres <ivalladolidt(a)terra.es>es>:
AFAIK you can do left channel minus right channel so
you're killing
whatever it's in the middle of the stereo field, which usually are
vocals. This work for karaoke. But I don't know a way to kill
everything except the middle of the stereo field, and I don't think
it's posible.
Reverse polarity on either channel (left or right, it doesn't matter) and
then do a LEFT-RIGHT and you'll end up with the signal that Ismael's
technique removed.
Sampo