This is just my two cents, but based on the number of people who currently use mp3 players during races, I'm guessing if they did strictly enforce this at Disney, two things would take place.
1. The would make 726 announcements and signs stating that they planned to do this and strictly enforce this, and you would hear more about it at the expo, etc, so there is no was no confusion about it and runners would be prepared.
2. The shear quantity of runners wearing mp3 players would cause this "enforcement" to create a huge traffic jam & commotion somewhere along the course (where ever it was that they decided to enforce this), so you would see and hear this commotion well before you came upon the scene. This would also be true if they tried to enforce it at the entrance to the corrals.
My thought is that there is no way they'll try to enforce this. I haven't seen it at any races since the rule went into effect, and I would imagine Disney won't be the first place that they will try it out.
Steve