Mixer
Audio Mixer details
Levels
In your system's audio mixer (Applications -> Sound & Video -> Volume Control) you can adjust various volume levels:
attachment:mixer.png
Of particular interest here are the microphone settings:
attachment:mixer_shaded.png
Below the volume slider is a mute button. If it has a red cross in it, your microphone is muted.
Mic Boost
Some soundcards also offer the ability to further amplify the microphone input, by way of a Mic Boost. In the mixer, go into the Edit menu and choose Preferences. Find "Mic Boost" in the list and click the tickbox next to it:
attachment:mic_boost.png
This should now give you an extra option in the "Switches" tab of the mixer:
attachment:mic_boost_switch.png
By enabling this, you may be able to correct for a quiet microphone.