http://linuxpoison.blogspot.com.br/2009/08/howto-convert-multimedia-files-between.html
https://trac.ffmpeg.org/wiki/Encode/MP3
http://stackoverflow.com/questions/7945747/how-can-you-only-extract-30-seconds-of-audio-using-ffmpeg
- https://gist.github.com/lukehedger/277d136f68b028e22bed
- https://gist.github.com/ksharsha/b06d184391290bc3b87fdadadb73c5bc#file-ffmpeg-compress
https://askubuntu.com/questions/442997/how-can-i-convert-audio-from-ogg-to-mp3
Concat audio: