The basic problem is that MIDI is not music. It's a record of the keypresses on the keyboard that entered the data into the computer.
In order to "convert" MIDI to Audio, something has to play the MIDI control file and then record the audio that was played.
The conversion programmes use the onboard sounds of the computer soundchip, which may or may not be anything like the instruments they are supposed to be.
So what you end up with is an audio recording of the sounds of your computer.
If you have a high quality soundcard, or if you use an external Sound Module that has high quality sounds, then the quality will be OK.
If not,you may not be able to tell acoustic guitar from marimba.
To sum up:
- MIDI is control data for a synthesiser
- WAV or CD is Audio to a reasonably high quality- everything on the original except the really high frequencies above hearing range is there.
-MP3,WMA etc is a thinned down sample of CD audio. It is those parts of the audio that the coding software thinks you really need to hear, ignoring the bits that you can do without.
Cheers
Dave