Hi
As said like trying to unmake a cake .
There are programs around like Audacity (free) that allow you to cancel the vocal out some what!
The effectiveness depends a bit on how much processing has been done to the original, vocal doubling etc
They all work basically by re recording the song shifting the recorded version out of phase and playing the 2 back together the resulting phase cancellation reduces the vocal frequencies
This is often used for making Karaoke backing tracks from recordings.
I have not found it very good, as it seem to cause a sqedging effect on the music around the mid range, and of course you have to re sing the vocal or remix it in from your source and rerecord again.

Sorry I cant be more positive frown
Have fun
Mike

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