This is definitely a difficult task, but there may be some hope! :)
Here's the little trick. You will need an external sound mp3 editor such as CoolEdit.
The trick is to rely on the fact that vocals are usually positioned in the "center" of the stereo field. (Balanced between the left and right speakers).
Unfortunately, there are usually other artifacts such as reverb and effects which are usually panned to one side.
You start with a stereo file and end up with a mono file with the center frequency cancelled. Its not very good for most songs...but it does work for some. This process is also known as the "OOPS effect" as it cancels frequencies.
Here's how its done with CoolEdit:
Load the WAV/MP3 file into Cool Edit.
Create a new window with no WAV file in it (File->New...).
Set the settings at 44.1KHz 16-bit *mono*.
Switch back to the original WAV/MP3 file (with the "Window" menu).
Select the entire left channel in the original WAV file. If you move the mouse to the top of the WAV display area, the mouse cursor gets a little 'L' next to it. Pick a spot near the middle of the screen, left click, and drag all the way to the left edge. Then move the cursor back to the middle, right click, and drag all the way to the right edge. You should now have the entire left channel selected.
Select "copy". Switch to the new WAV file, and select "paste". Switch back to the original.
Move the mouse cursor near the bottom of the WAV graphic until the mouse pointer gets an 'R' next to it, and select the entire right channel the way you did the left.
Select "copy". Switch to the new WAV file. From the Edit menu, select "Mix paste...".
Select "Overlap (Mix)", volume of 100, and check the "Invert" checkbox. Click "OK".
Give this a try to see how it works for you. You may want to just try looking around the 'net for an "instrumental" version of the song (if it is available).
Sourced from an Acoustica message board posting.