Are you on the 2.6.30.* kernel? (uname -r) Also make sure your microphone is not muted and you're capturing from the right microphone line in. To do so open KMix (if you're on KDE, otherwise the gnome
sound manager) and go to Settings > Configure Channels. And check all the microphone and line-in channels. Then play around with the settings of them (maybe you need to use the microphone ...