This is my script for screen-casting on Linux Mint 14. It records the desktop, computer audio, and the microphone, each on a separate track. To tie everything together I use OpenShot Video Editor.

gistfile1.sh

#!/bin/bash

arecord -D plughw:0,0 \
        -f S16_LE \
        -c1 \
        -r44100 \
        -t wav \
        latest-computer.wav \
        &

arecord -D plughw:1,0 \
        -f S16_LE \
        -c1 \
        -r44100 \
        -t wav \
        latest-microphone.wav \
        &

avconv -f x11grab \
       -s 1600x900 \
       -r 30 \
       -i :0.0 \
       -vcodec libx264 \
       -acodec libmp3lame \
       -ar 44100 \
       -preset ultrafast \
       -threads 4 \
       -f mp4 \
       latest-video.mp4