How make program for play Midi files on the FM syntetiser or ADLIB or PC SPeaker?

Hello,
it is possible in assembly, if you're good enough and if you take the time.
the easiest way whould probably be using some midi decoder dll, and go to windows.
well midi format is a litle like other trackers like mod, xm, s3m, uses instruments, loops ...
brief description about midi format:
http://jedi.ks.uiuc.edu/~johns/links/music/midifile.html
http://www.borg.com/~jglatt/tech/midifile.htm
http://improv.sapp.org/doc/misc/MidiFileFormat.html

some midi players with source codes:
http://www.filelibrary.com/Contents/DOS/38/22.html
http://www.synchrondata.com/pheaven/www/area21.htm
lookout with programmersheaven.com - there used to be viruses in archives, i suggest scanning all downloaded files with anti virus example : www.grisoft.com - AVG Free anti virus

adlib/soundblaster players withh source:
http://www.astercity.net/~malf/players.html

fm synthesizer examples/sources:
http://www.synchrondata.com/pheaven/www/area25.htm
adlib/soundblaster:
http://www.filelibrary.com/Contents/DOS/72/4.html
http://www.synchrondata.com/pheaven2/www/area8.htm

some misc docs:
http://alexfru.narod.ru/emiscdocs.html

you can find more i have just pasted here some nice looking sites.

i'd recommend you the xm format, i think its simple and quality, if you want to make some game or demo it is a good selection.

if you harvest the knowledge, you can make everything.