If the mic works in other programs, it should work in the game just as fine. Check that the microphone is selected correctly: http://www.mylittlekaraoke.com/forum/index.php/topic,682.msg2959.html#msg2959
After that, you might have to increase sensitivity by increasing the mic volume and tinkering with mic boost and threshold in the sound settings.
After that, you might have to increase sensitivity by increasing the mic volume and tinkering with mic boost and threshold in the sound settings.