In order to avoid always re-learning same things I’m listing few useful commands. For more detail and information go here.
Any video/audio to OGG/Vorbis
fmpeg -i source.file -vn -acodec libvorbis -aq 6 audio.ogg
The -vn option disables video recording — omit if source is audio file. The -aq option stands for audio quality. The value of 6 results roughly at 192 kbit/s. For other values check this table.
Any audio/video to mp3
ffmpeg -i source.file -vn -ar 44100 -ac 2 -ab 192 -f mp3 sound.mp3
The -vn option disables video recording — omit if source is audio file; -ar sets the audio sampling frequency; -ac number of audio channels; -ab audio bitrate in bit/s; -f forces mp3 format.
