MIDI is a collection of pre-made waveforms that are a standard, that come with a sound device.
To write a tune in MIDI you would merely need from 3 to 16 (I think it is now), to represent an instrument and what fashion u want it to play. Instrument being a pre-recorded sample.
Like a basic Yamaha organ, it`ll come with midi instruments, and also the ability to add to it by recording a sample and using it in the same way. I`m sure we`ve all at some point recorded our voice into a box and used a piano type keyboard to play it back at all kinds of cut-offs, periods, pitch, key. Well one midi instrument will be a triangle, a guitar, a piano, a drum, a handful of basic voice exclaimations... there are 1000s of them and are a recognized standard. The standard of which instrument is which number in the sample list is always the same. Sometimes the samples are different, but the same instrument.
There are different revisions too. The basic one which is like 2m, and a more up2d8 one which is 4 or 8m (can`t remember) - but it contains the 2m one at the start of it.
If you search for Final Fantasy VII midi tunes on the net, you can download them and play them through your pc. Oddly it`ll sound very very slightly different, but the ps1`s midi interface is exactly the same as the midi bank that comes with soundcard drivers, just the samples are a little different - but arranged the same.
It was a good idea, coming out of the 8bit age into the 16bit, but it always had the drawback of sounding "mechanical" to a degree, or too computerized.
I don`t see how these guyz are doing that, but then again I don`t know exactly what they`re doing. All I do know is the smaller you make music files, the lower the quality.