There have already been a couple of abortive attempts to build a midi interpreter library for flash.
It's not such a difficult process, seeing as you can embed midi files as octet streams, read their data at the binary level with the bytearray class and manipulate it however you wish.
The only issue is that there is currently no official way to load a sound object from binary data, meaning you would have to include samples of each instrument embedded in your swf, bringing back some (but not all) of the file bloat that midi would replace.
Most people simply use one of the hackish methods of "tricking" flash into loading a sound from a bytearray, but then become rapidly discouraged when reminded that building an entire library around an unsupported feature is just asking for the library to be broken by the next patch.
Magical-Zorse
ummm, you do know that midi files contain the building blocks of an audio file. Flash can't turn it into an actual audio file. Synthesizers for midi files are all different
aap
Yes, you need a MIDI synthesizer, but everyone has one build in their soundcard. And a MIDI may sound different for someone else, but that doesn't matter all that much.