Midi Format (GM) is a standard that defines how midi music is played accross
computer platforms or products. So it also classifies how each instrument
to be choosen out of a set of 128 intruments (from 0 to 127). When you play
midifile the binary information in the midifile tells your soundbank which
has to play a specific note (which preset number has to be used).
the links to trigger the instruments and listen how they sound.
If you have
a poor soundbank some instruments will sound very ugly - but if you have a wavetable
soundcard you can replace the sound with another soundfont.
marked with * are to be found here - see the download/soundfonts