SDL_mixer needs /etc/timidity.cfg as well as the timidity instruments in order  
to play MIDI files.  
  
To reproduce: "playmus /path/to/a/file.mid", results:  
 
    Opened audio at 22050 Hz 16 bit stereo, 4096 bytes audio buffer 
    Couldn't load [...]: /etc/timidity.cfg: No such file or directory 
 
If /etc/timidify.cfg is manually created without installing timidity++, the 
error message goes away, but there's still no sound (due to missing 
instruments, I gather). 
 
Installing timidity++ fixes both issues, so IMO SDL_mixer should have a 
dependency on it.  Leaving pulling it in as the responsibility of apps using 
SDL_mixer to play MIDI files would be suboptimal and fragile should SDL_mixer 
for some reason ever be changed to use native (GPL'd) MIDI internally.