I have found a bug in the jD3.9 Audio and reported it.http://www.jdownloads.com/forum/index.php?topic=11994.0
Whilst this is affecting the size of your player control bar that is not stopping the actual playing.I ran this on Firefox.
I downloaded SallyGoRoundtheRoses.m4r and used Convertio to convert to mp3 file. Made a new Download with the MP3 file and all was well.
So I think maybe the convertion of mr4 to mp3 was not good!
Just thought. Your downloadable file is an mr4 file - now the audio player does not support mr4. So you need to load the converted mp3 file using 'Select Preview File' setting and use that to upload the playable mp3 file. That is you have the mr4 as the downloadable and the mp3 as the preview.
ColinPS I have attached a mr4.png file that you can upload into flat1