These files accompany Bug 218. Here is the text:

When I tag m4a files with mp4tags (usually adding or modifying the genre) they
are no longer playable in mplayer.  It just skips right through them without
giving an error.  Sometimes it will play one that has been tagged, but in most
cases, it will not.  

These same files play fine in totem and on other platforms with other players,
so I don't think they have been corrupted.  

This problem has occurred for as long as I've been using m4a files--a few
releases back from Christian Marillat's Debian repository as well as the CVS
version today.  

file_works.m4a works fine.

file_doesnt_work.m4a is the same file after I have run

  mp4tags -g 'Rock'

on it.

The tagged file plays fine in other players.