I don't know if people are still having problems with this, but I came across a solution after reading other people's suggestions and playing around with it. I have a Mac. This is what works for me: I open the student's media file submission and click play. As the audio/video file is playing, I press CTRL while clicking the mouse on the upper left side of the video/audio frame. A menu appears, and one of the options is "Save audio as..." it will then download download as an mp4 file. I hope that helps!