Update - they all appear to be 'Microsoft ADPCM' files that my other audio tools have no problems with. MC and the stand alone APE tool can't read them.
As a test, I opened one in a WAV editor and saved it as normal PCM file. MC liked it, but Monkey's Audio stand alone (4.01) still gave an 'invalid input file' message. The APE converter in MC didn't seem have a problem with it.
