Thank you, louishen, but as I mentioned in my first post, VLC will not play the files either.
I booted from a FireWire drive with a pure version of 10.4.11 that I keep around for such purposes, installed Perian 1.1.4 and the files will not play there either, so this is not an issue of user modifications to the Mac OS X. (They played fine up until a few years ago when I switched to an Intel Mac, but I just don’t need to refer to them very often.)
QuickTime Player’s inspector displays the format as:
16-bit Integer (Big Endian), Stereo (L R), 44.100 kHz
MS-MPEG4v1 (MP41), 320 x 240, Millions
so it knows what they are and it plays the audio just fine.
Thoughts?
Thank you all for your troubles!