Most likely, the kinds of files you're trying to play are called TrueSpeech .wav files. They use the DSP Group TrueSpeech codec, which is missing in Windows Vista, 7, 8 and 10 (this codec used to be included in older versions of Windows).
The newest version of the WavPlayer will automatically install the codec, so you may want to uninstall your WavPlayer software and then download the newest version from our website at www.bytescribe.com/downloads.
If you're not using the Bytescribe WavPlayer, then you can download just the DSP Group TrueSpeech codec from our website at https://www.bytescribe.com/downloads/tsp_codec_install.exe.
After struggling to get some wav files to open on my Windows 7 system I was directed towards this simple to install codec. It worked perfectly. I am very grateful for this and for the information that the files might well have opened perfectly on older systems!