When I try to play certain files with my WavPlayer, "Error" appears and the files won't play.

If you try to play a particular file and the word "Error" appears in the bottom left corner of WavPlayer and the file won't play, it may mean that the file is not a compatible format.  If the file you are trying to play has a ".wav" extension, the file may require a unique CODEC that is not installed on your computer.  Also, verify that the correct file type is selected.  Select File | Open from the menu and choose the correct file type from the "Files of type" drop down list.