Well, it kind of works now. As Daniel T Chen posted 'linux-backports-modules-alsa-karmic-generic' and the alsa installed by the script Almozavr posted about are practically identical (l-b-m-a-k-g is a lot easier and faster to install though).

With alsa 1.0.21 'alsamixer is missing the (un)mute switch for the microphone output, this falsely led me to believe the microphone would unmute if I turned up the microphone output volume. After following  almozavr's sugestion in http://ubuntuforums.org/showthread.php?p=8235684#post8235684 I installed 'gnome-media' and ran 'gstreamer-properties' to change the sound input from pulse to alsa. It turns out this doesn't make any difference (it works both set to alsa and pulse), but 'gstreamer-properties' had a test button which unmuted the microphone. After installing 'gnome-media' and running 'gstreamer-properties' I also have 1 more input in 'alsamixer' now: 'Front Mic Boost', 'Capture' and 'Digital' where I only had 2 at first: 'Front Mic Boost' and 'Capture' IIRC (this still confuses me). 

With all inputs ('Front Mic Boost' and 'Capture') at their maximum the external microphone works (though it seems more noisy). The internal microphone works too, but it's to soft to be usefull. Well at least it works :)

In a few days I'll do a clean install of xubuntu again and check what step exactly makes the internal microphone "work".

After installing 'gnome-media' and running 'gstreamer-properties':
user@machine:~$ amixer 
Simple mixer control 'Master',0
  Capabilities: pvolume pvolume-joined pswitch pswitch-joined
  Playback channels: Mono
  Limits: Playback 0 - 64
  Mono: Playback 64 [100%] [0.00dB] [on]
Simple mixer control 'Headphone',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 64
  Mono:
  Front Left: Playback 59 [92%] [-4.00dB] [on]
  Front Right: Playback 59 [92%] [-4.00dB] [on]
Simple mixer control 'Speaker',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 64
  Mono:
  Front Left: Playback 59 [92%] [-4.00dB] [on]
  Front Right: Playback 59 [92%] [-4.00dB] [on]
Simple mixer control 'PCM',0
  Capabilities: pvolume
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 255
  Mono:
  Front Left: Playback 255 [100%] [0.00dB]
  Front Right: Playback 255 [100%] [0.00dB]
Simple mixer control 'Front Mic Boost',0
  Capabilities: volume
  Playback channels: Front Left - Front Right
  Capture channels: Front Left - Front Right
  Limits: 0 - 3
  Front Left: 3 [100%]
  Front Right: 3 [100%]
Simple mixer control 'IEC958',0
  Capabilities: pswitch pswitch-joined
  Playback channels: Mono
  Mono: Playback [off]
Simple mixer control 'IEC958 Default PCM',0
  Capabilities: pswitch pswitch-joined
  Playback channels: Mono
  Mono: Playback [off]
Simple mixer control 'IEC958',1
  Capabilities: pswitch pswitch-joined
  Playback channels: Mono
  Mono: Playback [off]
Simple mixer control 'Beep',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 31
  Mono:
  Front Left: Playback 31 [100%] [12.00dB] [off]
  Front Right: Playback 31 [100%] [12.00dB] [off]
Simple mixer control 'Capture',0
  Capabilities: cvolume cswitch
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 46
  Front Left: Capture 46 [100%] [29.00dB] [on]
  Front Right: Capture 46 [100%] [29.00dB] [on]
Simple mixer control 'Digital',0
  Capabilities: cvolume
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 120
  Front Left: Capture 120 [100%] [30.00dB]
  Front Right: Capture 120 [100%] [30.00dB]