This problem has been plaguing me for a while. Running Ubuntu 8.04 on my Toshiba satellite laptop (p200-EE), when my girlfriend wants to call her family back in New Zealand on skype the microphone never seems to just work - the skype test call always returns nothing.
After hours of testing I have compiled a very specific solution to this problem - finally!
Play around with the settings!
Until I work out exactly what it is that I do during all of my scientific testing that makes the microphone work I'll jot down some of the things that I think make the microphone work :-)
Try opening the volume control (double click the speaker in the top right corner of the screen) and then Edit->Preferences
Tick everything, especially stuff like 'Capture' and 'Capture 1', and stuff like 'Input Source'. Then close that and there should be a bunch more volume controls available.
- On the capture tab you want to make sure none of your capture devices are muted.
- On the options tab there may be a dropdown with values like 'Mic', 'Front Mic' and 'Line'. Its a good idea to test these.
- On the playback tab there may be some controls for 'Mic Boost' and 'Front Mic Boost', get the boost up there a bit.
If playing around with these settings still doesn't work with the skype test call then try System->Preferences->Sound. The setting for 'Sound Capture' is a good one to try setting to something different, although if its set to ALSA thats usually a good option.
So to sum up, I don't really know how to solve this problem but that solution is about as good as I can find on the internet. Find a how to on installing skype on Ubuntu 8.04 here.