Might not help, but worth checking the fundamentals...
If, in Messenger, you select the Audio/Video Tuning Wizard, when you get to the choice of microphone sources, do you see your soundcard device as the chosen option? If so, when you proceed through the test, does Msngr warn you that your mic isn't enabled? Does the little bar graph move at all when you speak into the mic during the wizard's test?
On a similair note, if you double click the speaker icon in your system tray, is the Mic muted, and if there is an advanced button, is AGC (or similair option for controlling/boosting gain) ticked? Can you hear the mic if you turn your speakers right up? Also check the equivalent settings under the record section - Options menu, Properties, Adjust volume for recording.
If you don't have a speaker icon in your system tray (haven't had SB since the SB16ASP), goto Control Panel, Sound and Audio devices (or Sounds, Speech & Audio devices, then Sound and Audio devices if you use the category view in Control Panel), click the Advanced button.
All above assume XP is your OS, should be fairly similair anyway...
Have you tried other mics, and have you tried the same mic on someone elses PC/amp?
If you are using some form of extension from the soundcard (be it your case's front panel audio connectors, or a SB 5.25" audio breakout bay type thing), have you tried the other sockets?
Does the LineIn input work OK? You don't need anything fancy, or even necessarily line level output for a quick test.
Sorry if no help, but worth a quick run through....
Anyone else got a suggestion?