I created a neat CMS with audio uploads and multiple jplayer HTML5 audio players on a page (as opposed to a playlist) all seemed well until the user started uploading m4a files – my logic has presumed mp3 only and I had hardcoded mp3 into my player script for each file – after changing this to be dynamic and actually respect the original extension, I was still baffled why jPlayer refused to play the m4a files in Firefox (worked fine in Chrome) – then the penny dropped about relative paths to the media files! I had been using these without problem on the mp3s but m4a files need an absolute path or url to work in Firefox (and probably other browsers). Hope this helps other head scratchers out there….