Some examples and tips to convert formats and record audio/video using avconv tool

avconv-usage-tips

# ---Format conversions
# Extract audio from video file
avconv -i video.mp4 -vn -f mp3 audio.mp3

# Extract just video without audio
avconv -i video.mp4 -vcodec libx264 -an -f mp4 just_video.mp4

# Convert MP3 to WAV
avconv -i audio.mp3 audio.wav

# Convert AVI to MKV
avconv -i video.avi -vcodec libx264 video.mkv

# Convert MOV to MP4
avconv -i video.mov -c:v libx264 video.mp4

# Convert MP4 to AVI
avconv -i video.mp4 -vcodec libx264 video.avi

# ---Screencasting/recording
Note that the input -i option varies depending on your system specifications. 
Use pactl list sources and look for the "Name" string to look for the operative audio device in your computer
# Record desktop video/audio
avconv -f pulse \
        -i alsa_output.pci-0000_00_1b.0.analog-stereo.monitor \
        -f x11grab \
        -r 30 \
        -s 1920x1080 \
        -i :0.0 \
        -vcodec libx264 \
        -preset ultrafast \
        -threads 4 \
        -strict experimental \
        -y myscreencast.mp4

# Record just audio
avconv -f pulse -i alsa_output.pci-0000_00_1b.0.analog-stereo.monitor audio.mp3