Back when I was a teen, I used to strip out the vocal tracks on stereo music, by disconnecting the speaker ground wires from the back of the amplifier and connecting them to each other (so each speaker would ground against the other one). Since the vocal was "center", it had the same waveform on both speakers, so the speakers couldn't make a sound if they couldn't dump the ground to something. And the instrumentals weren't distorted too much. At least it worked good enough for a cheap Karaoke setup.