Shortly thereafter, a Boing Boing reader who worked on the Amazon MP3 program said, "we don't use 'proprietary' anything," and vowed to get to the bottom of it
Now the mystery is solved!
It turns out that the user had a file-permissions problem with his Amazon MP3s -- and that there's nothing proprietary on Amazon MP3s after all:
Thanks to some help from an Amazon employee in the Boing Boing comments, we figured the initial customer service rep who told me "it's proprietary" was misinformed. This led me back to poking around on my system, at which point I discovered the mp3 files only had read permissions for my user account, when I made them readable for other users, both mpd and mt-daapd were able to find them.This is totally awesome -- thanks to all the BB readers (and especially "AmazonMP3") for getting to the bottom of this!