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AFAIK, you can't mute the microphone in the car. This is something that may "cure" itself with a restart (hopefully...), or perhaps the microphone got disconnected somehow. It is located in the front overhead console. To test the microphone's operation and sensitivity, enter the navigation diagnostics: Follow the directions here: http://rangerovers.net/forum/viewtopic.php?f=5&t=7209 - but instead of the 1791 bypass code for navigation in motion, enter code 753 and press OK. You should see the navigation diagnostics's main menu. Press "Manual Checks" and "Microphone Test". Speak toward the overhead console in a normal tone (lower the radio volume first, if the radio is on), if the mic can detect your voice you should see the sensitivity bar graph moving up and down along with your voice.
To exit diagnostics, press the Return arrow in the upper right corner of the screen until the monitor prompts you to recycle the ignition, then remove the key.