Front panel audio headphones/microphone

cat /proc/asound/modules

Hopefully you will see the following:
snd_hda_intel

Now run:
aplay -l

This will tell you the type of sound card you have. In my case:
[ALC888 Analog]

Now gunzip (or zcat | more) the following file, and scroll down until you see your card:
/usr/share/doc/alsa-base/driver/ALSA-Configuration.txt.gz

This tells you which 'model' you can use. In my case I found this (first few lines only):
ALC883/888
3stack-dig 3-jack with SPDIF I/O
6stack-dig 6-jack digital with SPDIF I/O
3stack-6ch 3-jack 6-channel
3stack-6ch-dig 3-jack 6-channel with SPDIF I/O

Now modify:
/etc/modprobe.d/alsa-base.conf

And add the following to the end of the file:
options snd-hda-intel model=6stack-dig

You should obviously choose the correct model for your sound card.

After a reboot I used xfce4-mixer and found the headphones, microphone and more!