Well, all 4 failed. In fact two didn't even make it to into the feed with both the Audion and the Logic Pro mp3 files disappearing into the ether. And yet all 4 work fine in the audio player right on the page.
One suggestion is that the problem could be the hosting server not understanding what kind of file an .mp3 is. To test this, I'm going to try the Logic file again, but this time I've added the line - 'AddType audio/mpeg mp3' into the htaccess file at the root of the site.
Here goes nothing...
JB MP3 test Logic
Nope - looks like there's nothing there.
BTW - if you want to play along, the 'feed' is this url:
Just copy and paste this into the W3C feed validation page
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.