It turns out that the mp3s were fine. As best as I can tell, the server hosting the files compresses the data it sends to browsers - even mp3s (uses gzip). Some browser/plugin combinations misinterpret gzipped responses, causing playback to stop after 5 seconds or so.
I was hoping to be able to post a work-around here for users encountering similar problems. But, no luck thus far in overriding my hoster's settings for my files.
Instead I moved BitPotion mp3s to my own server. The connection is slower, but I have full control
Anyway, the mp3s at
http://bitpotion.com should play now. And, as always, thanks for listening.