How should unused pins be handled on audio codecs?

There separate rules to follow for unused analog audio and digital audio pins:

Analog audio input pins (i.e. inputs to the ADCs) that are not being used should be connected to the common mode pin or filter.

Analog audio output pins (i.e. outputs from the DACs) should be left unconnected (NC).

Digital audio input pins (i.e. I2S port pins: BCLK_IN, LRCLK_IN, SDATA_IN) should be connected to GND.

Digital audio output pins (i.e. I2S port pins: BCLK_OUT, LRCLK_OUT, SDATA_OUT) should be left unconnected (NC).

If an MCLKI (master clock input) pin is not used, it can be connected to GND.

If the MCLKO (master clock output) pin is not used, it should be left unconnected; typically, the MCLKO pin can also be shut off in the chip's control registers.

In the case of the AD193x family, any unused ADC analog input ports should be connected directly to the CM pin, along with the recommended decoupling capacitors. If an ADC port is being driven with a single-ended source, the unused pin of the differential input should be AC coupled to ground with a 10 uF capacitor.