ffmpeg

extract audio from video

ffmpeg -i input-video.mkv -q:a 0 -map a output-audio.mp3

remove audio from a video

ffmpeg -i video.mkv -c copy -an video-nosound.mkv

record a video

without audio

ffmpeg -f x11grab -r 15 -i :0.0 -acodec libmp3lame -vcodec mpeg4 -ar 48000 -qscale 0 -framerate 24 outputvideo.avi

with audio

ffmpeg  -f alsa -ac 2 -i alsa -f x11grab -r 15 -i :0.0 -acodec libmp3lame -vcodec mpeg4 -ar 48000 -qscale 0 -framerate 24 outputvideo.avi

convert video format

ffmpeg -i videofile.mp4 videofile.webm