For everyone who download a webm Video and want to convert it to mp3.
You need ffmpeg.
Source: https://bytefreaks.net/gnulinux/bash/ffmpeg-extract-audio-from-webm-to-mp3
Example: cd ~/Videos
then call your script(in my case webmtomp3.sh)
that is all
If you have mkv files in Videos you have to put it in another place, when you call that script.
#!/bin/sh
#
# convert all webm video files to mp3
#
#
#
for FILE in *.webm; do
echo -e "Processing video '\e[32m$FILE\e[0m'";
ffmpeg -i "${FILE}" -vn -ab 128k -ar 44100 -y "${FILE%.webm}.mp3";
done;