A midi file does not contain sounds at all, it is a musical score, and can contain up to 16 instruments at once. A well done one includes many nuances, including pressure on keys, sustains, and chords. This is one limit to how good a midi file can sound.
Another limit is the Synth (in your soundcard or a software synth) that plays back the midi file.
If midi interests you here are two programs to fool around with.
1. This is a free program that shows what is in a midi file, and allows simple editing of the score.
http://www.anvilstudio.com
2. This is Wingroove, a shareware software (less than 1.44 Mb) synth . Version 9E You can use it stand alone or make it your default midi engine.
http://www.cc.rim.or.jp/~hiroki/english/wgdl.html
I bought this about 6 years ago. It was free for about 30 days at the time, and $15 to register. Use this to play your midi files and see how much difference there is!
It allows you to play back midi files and change on-the-fly individual instruments. It's reeds are
very good as are several of the horns and pianos.
Almost all of my midi bookmarks have gone now, so that's the best direction I can give at this time. If you enjoy these programs and need further help , post here and I'll try.
Jim