Hi, this means that your WAV files are the wrong format... WAV files have to be, um, 8 bit or 16 bit (I can't remember which), and if you use the wrong one then you get this error. I use the Wave Studio that came with my SoundBlaster card to convert .wav files to the correct format. You should be able to find something that will do the job at CNET.
Cheers,
Keith