How to use an audio codec as an ADC?
You just need to scale the voltage and adjust the DC offset (if needed).  There is an important thing to know when doing this, however.

Audio ADC's often have high pass filters, two of them actually.  The first is a cap in series with the signal path.  The second is a digital filter that is inside the ADC.  These filters usually remove anything from 0 to 10 Hz, but sometimes it could be as much as 0 to 200 Hz.  Sometimes you can disable these, but usually not.  If you are using the ADC for non-audio this might be a problem for you.