Challenge: figure out Amazon's crazy-ass "proprietary" MP3 tagging system -- UPDATED

Be sure to read the updates below for the solution to the mystery!

Pete sez,

As an experiment, I recently purchased a couple of albums via Amazon's MP3 Downloads store. I figured since they've gone DRM-free and all the majors are on board, I ought to at least give them a try.

Aside from the fact that full-album purchases *require* use of their pointless MP3 Downloader software (no Linux version, of course, so I had to fire up XP in a virtual machine), the experience wasn't so bad.

However, I use mpd (http://www.musicpd.org/) and mt-daapd (http://www.fireflymediaserver.org/) to share out my music on my local network, and both rely on background scans of ID3 tags. Unfortunately, neither service was able to pick up the ID3 tags on the mp3's I got from Amazon, even after I manually re-tagged all the tracks.

I contacted Amazon customer support, and got the following response:

"Indeed this is a little more of an abstract question. Unfortunately we are unable to release any information on how we run our ID3 tagging as it's proprietary. The only thing I can think to do is manually re-tag each Album/Track.

I hope you find this information helpful. Thanks for buying MP3 Music Downloads on Amazon.com."

Proprietary ID3 tagging? Seriously?

I've continued to fiddle with these tracks, so far to no avail. The weird thing is, local clients like Rhythmbox and Amarok (as well as my ID3 tag editor) have no problem seeing the tags.

So, who knows how this works? Can someone please reverse-engineer the Amazon tagging scheme and add notes to the comments? Let's get this figured out, folks. (Thanks, Pete!)

Update: In the comments, Amazon MP3 sez,

I apologize in advance for the response from Amazon customer service. Don't know exactly where their info came from (they're usually totally on top of things), but we don't use "proprietary" anything. "Crazy-ass" sometimes, but not proprietary.

We worked very hard to make sure the tag info we place in the MP3 files works everywhere (don't get me started on how different programs choose to interpret the "Ensemble" tag). Up till today we were doing pretty well; no complaints at all.

Obviously we've optimized the tags to work best with iTunes and, ahem, WMP, but they should work everywhere. If you've got any more detail around what might be confusing mpd and mt-daapd, post away. In the meantime, I'll look into it.

Update #2: Mystery solved! It was a file permissions bug:

Thanks to some help from an Amazon employee in the BoingBoing 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.
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