If you set the Xbox Dashboard > Live > Voice setting to "Play Through Speakers" can you hear the others talking to you? If the chat talkback works that way, but doesn't work when you have it set to "Play Through Headset" then you probably have a failing chat-controller cable. If you replace that, the problem should be fixed.
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If you set the Xbox Dashboard > Live > Voice setting to "Play Through Speakers" can you hear the others talking to you? If the chat talkback works that way, but doesn't work when you have it set to "Play Through Headset" then you probably have a failing chat-controller cable. If you replace that, the problem should be fixed.