It can't be done - sorry. You can get quite good results by very carefully passing the audio through a long series of filters at different frequencies - but you're also going to be removing the music at those frequencies.
Boss do a pedal for guitarists called (i think) a centre canceller which takes out lead guitar parts to allow you to play over the top of original tracks,or isolate them to learn them. This also has the effect of removing vocals which are also recorded in the centre of a stereo mix (along with any other instruments taking a solo). This may suit your purposes though as i've never used one i can't testify to how good they are.
AnalogX have an App. called vocalremover. Freeware. I downloaded it but haven't had chance to try it yet. AnalogX do some good sound apps. Their freeware Vocoder works great. Also their text to speech app. called Sayit. So maybe they got something like.. Also I am not an Analogx salesman.