Instructions
Unmute the Microphone
Turn on your Xbox and press the "Guide" button on the controller.
Scroll right to the "Settings" tab.
Select "Preferences" and "Voice."
Highlight "Mute" and press the "A" button to deselect it.
Scroll to the volume bar. Press right on the left stick or D-pad to increase the headset's volume.
Unmute on Xbox Live
Power on the Xbox 360 and press the "Guide" button.
Press "Y" to access the Xbox Dashboard.
Select "Friends" and scroll to the "Players" tab.
Select a player's gamertag. Scroll to "Mute" and press "A."