Unused analog inputs can be left open/unconnected in most typical cases. AC coupling unused analog inputs to ground is an extremely conservative solution, and does come at the cost of area and additional components. However, if you have an extremely noisy environment (lots of digital blocks / nearby traces, frequencies on the board near the ADC sample rate [fs(ADC) = 64 x fs(audio)], etc.) AC coupling might improve performance. In most systems, simply leaving the unused analog inputs unconnected is acceptable, as long as you are confident the software will never accidentally enable them. If you have the space and wish to use a conservative compromise solution, having the AC coupling pads available but not populated by default is an option as well.