Hey yourself:
Is this only with ONE particular file -- or are we talking about a problem opening ALL MP3 files in WavePad?
If it's only one particular file, my guess is a damaged file. Here are some things you can try to repair the damaged file.
A free MP3 repair tool:
http://www.drtag.de/en/description_mp3rt.htm
-OR-
Try opening it in MP3DIRECTCUT (freeware)
http://www.mpesch3.de/
If it plays there, SAVE it with a new name. Maybe that will write new file headers on the copied file, which might then play better in the other applications.
Good luck.
Jerry