XBox 360 - unable to join chats

We have an Xbox 360 that is hooked to a switch in one room, switch is hooked to our router. We can join parties, but they can't hear us. We can hear them. The mike is plugged in, but it won't light up. We can record voice messages to send, but can't chat in private or party chats.

Any ideas on what to do? We have been messing with it for a few weeks and nothing is working.

Thanks.

Nancy