Midi files aren't like ordinary sound files. They are in effect a series of instructions to your soundcard such as pitch, duration, volume and instrument. This is what allows midi files to be so small compared to a standard sound file.Although there have been several attempts to write software to convert from say wav to midi the results are pretty hopeless. Just imagine how clever a program would have to be to be able to pick out each individual instrument in a piece of music then create the instructions (midi) to get your soundcard to reproduce the right sounds.