So, with two of the files failing even to be included in the feed at all, and two of the files being included but faulty, I'm going to continue the tests with the faulty ones. No point experimenting with invisible files really is there?
So, with the htaccess tweak in place, we're going to try the iTunes mp3 again.
MP3 Test iTunes
Ah - now that one seems to have disappeared too. Sheesh.
Update: in the end, there was nothing wrong with the mp3 files, but there was an issue with the server. See Podcast tests - conclusion for the full lowdown.