Swordy posted about this in the pit a few months back and I have run into a few others cases of this...

Any way to audit the database and and see how many or which ones are missing?  It is kind of a bummer to suddenly have the system losing audio files.

Actually it looks like the post Swordfish linked was removed at the end of the day.  Here is another example I noticed from clicking around.  Happens to be mine:

http://forum.bandamp.com/Audio_Review/9412.html

I'm looking into this issue. We have a number of backups made over the years and I'm going to dig through them to see if I can recover the missing files.