Well, for making your own IDF it really depends on what you want to do. Sometimes you can use pre-existing ones and just get a license to use them in a game(like the soundbanks from Sound Modules, or synethesizers like the SC-88).

Then there are the ones you make yourself, by using a bank format like DLS, SF2, SBK, etc. There are hundreds of different IDFs you can use and for the MIDI to playback using them, it''s just a matter of the programmer making a playback device that uses the sounds from the given bank you decided to go with. SF2 files are easy to make and use, but they are only supported by Soundfont Compatible soundcards and I don''t think any consoles to this date have the proper drivers to use soundfonts either.

The DLS file(which stands for DownLoadable Sound) is directly related to DirectX and DirectMusic Producer. Anything that supports DirectX can use DLS banks for playing back MIDIs. There are tutorials on Microsoft''s site for how to do so too.

Still, I wouldn''t advise sticking with MIDI alone. While MIDI might be versatile, there are some consoles/systems like the GBA that can''t handle MIDI as well as some other formats such as Module Formats(MOD, XM, S3M, IT, SPC, NSF, etc).