To make sure that you can easily control Xbox Live chat volume, please make 
sure that the following settings are configured on your Xbox (These are the 
default Xbox settings).

1. From the Xbox 360 Dashboard, press the Xbox Guide button (the X button in
   the center of your controller) to access your Xbox Live Settings.
2. Navigate to the "Personal Settings" selection, and press the "A" button on
   your controller.
3. Select "Voice" from the following menu, and press "A".
4. On the Voice screen, make sure that "Play Through Headset" is selected for
   Voice Output selection and that volume is set to 10.
5. You can fine tune game audio and chat volumes using the X3 amplifier and
   chat volume controls.

  

If your Xbox 360 is connected using HDMI only (not the red/white RCA plugs),
and the 'talkback' does not play through your headset, try changing the Xbox
360 Dashboard Personal Settings to "Play Through Both" as in the picture
below:

   

NOTES: 
1.  This may interfere with your ability to individually adjust the levels 
    of the game soundtrack and Live Chat. 

2.  Make sure to keep the volume control on the puck turned all the way 
    down, so you don't get an unwanted echo.
______________________________________________________________________________