>>3
Karaoke machines use instrumental versions of original music that is re-recorded by either a) the original producers of the music, or b) a group whose job it is to take sheet music of the original music and recreate it without the lyrics.
On Japanese singles (and from what I hear a few American ones, though I've never seen them myself) you will see an A-side, a B-side, the karaoke version of the A-side and the karaoke version of the B-side.
When you buy Karaoke CDs you get basically a CD full of instrumentals. There is no way of removing the vocals because to a computer they are just more sound and mixed completely into the waveform.
It's like taking an image macro with text written all over it and trying to run it through an automated program to get rid of the text. It can't be done, because by the time the final product (the image) has been created, the pixels are all mixed together and whatever was behind the text has been irreparably lost unless you either know what was there before and can redraw it or if you have the original PSD file and can just remove the text.
By the same token, producers mix the output from vocals, mike, drums, and guitars and synths all together from separate inputs and then spit out a fully blended-together soundwave. To a computer, sound is sound without any special meaning.