If you're using any variant of a SoundBlaster X-Fi... Well, I'm sorry. It's a known issue they refuse to fix and can only be addressed by downgrading or upgrading drivers at random or repeatedly plugging in and unplugging the mic. No elegant solution.
If you DON'T have a card from Creative, how exactly is it not working? When you talk, does anyone report that their sounds or other voices stopped? TF2 and other Steam games have an issue where a microphone that is turned up too high using the in-game settings will clip and become muted, while silencing other voices or sounds (the combination depends on sound card). Try turning your output down one or two notches on the in-game Voice options panel. If that doesn't fix it, are your drivers up to date?