It's almost impossible. Think of the audio like a cake mix and the vocal as the egg. now you have the mixed and baked the cake and now you want to get the egg back. See?
You can have some success REMOVING the vocal (to some extent) with an audio editor. A free one like Audacity will do.
This kinda works IF the vocal was a mono vocal and is sat right in the centre of the sound stage. What you do is copy the track. Invert the phase of the copy (most decent editors allow this) and sum the two tracks together.
Now anything right in the middle of the sound stage will cancel out and anything else won't, so theory has it the vocal disappears. Problem is, most recordings use stereo reverb on the vocal and that won't be in the middle and won't cancel out so you get a ghost of the original vocal.
If you do that markone the whole track will disappear.
If you get a signal like a sine wave, and a copy of that sine wave, invert the phase of one and put them together - they cancel out. Same thing happens here.
you need to make sure it is stereo to be able o do that as the vocals are normally in the centre and the music spread out.
You can also take the leads from your speaker and put one lead in the left +ve of the amp and the other on the right +ve and that will do the same, but I take no responsibility if it does not like it. only the difference audio will be able to be heard, I use to use it as a poor mans surround sound.
Seprate Vocal From Audio Songs 
Hopefully a long day of doing fun stuff like making movies.!
Linear Mode
