You can first view your current default audio device by typing:

pactl stat

And with

pactl list

you can view all your devices.

And with this command

echo "set-default-sink alsa_output.Headset" | pacmd

you can set the default device.

But I found a much better source for this problem and a included little shell script for this problem here http://ubuntuforums.org/archive/index.php/t-1370383.html