Carnegie Speech recommends a USB headset with the flexible boom and a noise-canceling microphone. This is especially important in a language lab setting or any noisy environment (dorm room, LRC, etc). The built-in microphone in your computer will not work properly.
External Article: (Macworld) Microphones for speech recognition
Recommendation: Microsoft LX3000 USB headset - low cost, great performance with NativeAccent in a classroom, learning lab, etc.