Thanks. I wasn't the one who extracted the midis though, I just found them on a forum.
Midi files are a bit like sheet music, they say which instrument plays which note, for how long, how loud, with how much emphasis and so on. The problem is that most standard synthesizers (for example the Microsoft software synth) come with low-quality instrument samples. There are better sample collections out there (often in the soundfont format), but the good ones are usually big. The main soundfont I used is the new beta verison of Shan's General Midi font called SGM-V2b, which is around 250MiB.