EnableMic

According a Mr JMills:

sudo gainroot
cd /sys/devices/platform/audio-i2c/
echo 1 > mic_enable
echo 1 > mic_loopback
echo 100 > gain_left
echo 100 > gain_right
echo 90 > volume_left
echo 90 > volume_right

will loop the microphone into the speaker/headset system, so you can test it by wearing a headset. Note that the loopback switches itself off when the display dims, so must be re-enabled.

Make sure you use headphones in the tablet's headphone jack when performing the above test, as otherwise it will likely go into a feedback loop if you use the internal speaker, potentially damaging the tablet!

Presently there is no known driver for the audio input, so Linux apps can not use the microphone... yet.