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).
