Thank you, but what's MOD? I never heard of it. Sound interesting though.
MOD is similar to MIDI in the sense that it stores only the notes being played and their attributes and effects applied to them instead of recording the entire waveform. But instead of relying on the environment to supply the instruments, the instruments (and their specific sounds/waveforms) are embedded in the file. There are a number of related formats generated by various "tracker" programs which create and edit these MOD files and their relatives. Most or all of the sample music provided with SGDK2 is in MOD format or a related format. I think MODPlug
looks like a pretty impressive free tracker, which I was using to preview all the SGDK2 music before importing it into a project template. It supports MIDI, MOD and many, many other formats. I think most of the music that I added to the music library was in IT format or XM format, which are a pretty flexible/powerful relatives of the MOD format introduced by Impulse Tracker and FastTracker respectively.
I went to some site like http://modarchive.org/
to get a few of the files in SGDK2's music library, but was only able to get permission from a couple of authors. Of course you can use whatever you want personally (or get your own permission from authors of whatever music you want). I really like the relatively small files that sound really good, of course.