I'm trying to use the Firefox Add-On Downloadhelper to download and convert this 'video' of Einstein: http://sploid.gizmodo.com/listening-to-this-incredible-recording-of-einstein-will-1464557213 in order to have only its audio. Unfortunately the conversion fails and I have the following information from the console.

Notes: I opend firefox with sudo

Executing /usr/bin/avconv -i /tmp/gizmodo_2249.mp4 -y -v 0 -strict experimental -ab 192k -f mp3 /home/jk/dwhelper/gizmodo_2249.mp3

The result was a 0 kB mp3-file.

Update:

I've decided to sudo-open firefox and remove VideoDownloadhelper. Then I opened Firefox as a nomal user with the terminal and installed the Add-On...Problem persists...Now I'm learning to use avconv.