When previewing an mp3 file there are 2 fallbacks. First if the browser
supports mp3 playback natively via the html5 audio tag, then this is used
(chrome and ie). In the case of firefox which only natively supports ogg, we
fallback to a flash solution. In your case you probably are missing flash so
there is no fallback. Ogg files should work for iceweasel/firefox,
unfortunately, there is not much we can do for mp3 files.