Um, interestingly enough there is a selection under Effects -> Special Effects called Reduce Vocals, but it has a rather serious limitation:
As the Help file says (note the last sentence, bad grammar et al),
If you want to reduce the vocals from a music track you can use this effect. From a stereo recording, WavePad will attempt to identify the voice in the left-to-right spectrum and remove it. The recording must be stereo (from an original stereo source like a CD). It will also remove any instruments near the voice in the stereo spectrum.
Note: it is impossible to remove the vocals perfectly without the original mix track. You will notice some instruments might be removed too and some vocal remain. The effect will also not work on some files which have previously encoded in a highly compressed form like mp3 (because this remove some stereo depth).