Instructions
Make sure the headset connector is completely plugged into the socket at the bottom of the Xbox 360 controller. If it is only slightly loose it may not work properly.
Check the mute button to ensure you haven't accidentally muted yourself. This button is located on the headset connector. Push it to the left to mute your speech and to the right to use the microphone.
Read your game manual. Some games require you to push a button to speak through the headset's microphone.
Adjust the microphone's position. Microsoft recommends that users place the mic one inch from their mouths and slightly off to the side to prevent transmission of breathing during a chat session. Make sure the mic's opening is pointed toward your mouth and not away from it.