RE: how other people creating MIDI/mp3 teaching files have tackled this problem.
ANSWER - We do it with software.

RE: perhaps creating a series of perhaps 2 or 3 mp3 files for each tune.
WHY are YOU trying to teach? Heaven help your protege'

The standard pedagological approach is: Whole - PART - Whole
The student hears the WHOLE
It is broken down into PARTS and learned
The piece is practiced until it becomes WHOLE at the correct tempo.