Hey, its usually because the WAV is not 16bit. You must convert all the files that fail (or simply all of them :P).
You can download LameXP for free: Download LameXP
And can also use (my favorite) Audacity for free: Download Audacity
Use Audacity if you want to edit the sound to your liking before exporting to Wav 16bit.
If no editing needed, use LameXP to batch convert a bulk of files together in a single conversion.
Enjoy