The secondary or upper mic on the iPhone 4S is located at the top of the phone next to the headphone jack. There is a small pinhole you can see from the exterior of the phone. On the interior, this mic is located on the headphone jack flex cable. You can test it by using the speakerphone function. iPhone 4S Headphone Jack Cable Replacement
I've had this problem on the occasional 4/4S LCD, and suspect that some LCD flex just can't handle bending prior to installation. You may want to test with an extension cable, or just wait until the LCD is installed. It's also good to avoid connecting/disconnecting the LCD while the backlight is on.
What size drives will fit?