Instructables
This will teach you how to remove the vocals from almost any song.
This is great for making your own Karaoke song

Now before i start i want you to know this won't completely remove the singer, but it will do a pretty good job of it so its worth a try.
 
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Step 4: Invert Right Audio

For this step you need to invert the audio in the right bar.
To do this you need to click somewhere on the gray part of this bar,
then go to the Effects menu and select invert. (be sure the song is not playing when you try to do this)
The right track should look only a little different.

Step 5: Change To Mono

Change both the right and left tracks to mono.