http://photosounder.com

Starting a song (Britney Spears' Womanizer) and its instrumental version, we open both in Photosounder, export the resulting images to Photoshop, paste the instrumental's image onto the full song's image, adjust levels so they match in brightness, then set the blending mode to Difference so that only the features present in the full song but not in the instrumental are left.

We then save the resulting image and open it with Photosounder.