MacOS – Internal Mic stopped recording voice, but hears case/keyboard touching

I'm delighted to report that, after installing the OS X Mountain Lion v10.8.3 Update, this problem has gone away:

About the update

This update is recommended for all OS X Mountain Lion users and includes features and fixes that improve the stability, compatibility, and security of your Mac, including:

[ deletia ]

• A fix for an issue that may cause the screen to display incorrectly after waking from sleep

KeyRemap4MacBook also supports device-specific settings:

<?xml version="1.0"?>
<root>
  <devicevendordef>
    <vendorname>KINESIS</vendorname>
    <vendorid>0x05f3</vendorid>
  </devicevendordef>
  <deviceproductdef>
    <productname>ADVANTAGE</productname>
    <productid>0x0007</productid>
  </deviceproductdef>
  <item>
    <name>modifierkeys</name>
    <identifier>modifierkeys</identifier>
    <device_only>DeviceVendor::KINESIS, DeviceProduct::ADVANTAGE</device_only>
    <autogen>__KeyToKey__ KeyCode::CONTROL_L, KeyCode::FN</autogen>
    <autogen>__KeyToKey__ KeyCode::FN, KeyCode::CONTROL_L</autogen>
    <autogen>__KeyToKey__ KeyCode::COMMAND_L, KeyCode::OPTION_L</autogen>
    <autogen>__KeyToKey__ KeyCode::OPTION_L, KeyCode::COMMAND_L</autogen>
  </item>
</root>

You can see the vendor and product IDs from EventViewer.app. If it can't remap any keys on the external keyboard, try uninstalling other keyboard or HID drivers.

See the source for the key code values and predefined settings.