You need a more sophisticated music program that can play samples, which are simply small snippets of recorded instruments which can be looped by a music program to sound like a real instrument playing real notes. For example, Cakewalk, Sibelius, etc. are music programs that can play these samples over your midi notes, so instead of hearing the archaic MIDI notes (which, by the way, is your computer's soundcard's "samples", though they arent real instruments, they just try to sound like them) you would hear nice, realistic instrument sounds. Of course, you also need the samples themselves, which are available alone on cd's (search for East-West Symphonic Orchestra, for example), or available with the music software itself...
GameMusicMaker