We used AUHAL's channel layout to determine the number of channels on both output and input. This was broken for input, see  issue 794501 .

When fixing that issue, I couldn't find that channel layout should be possible to read from AUHAL's input element (i.e. for input/capture device). On the other hand, I couldn't confirm that it cannot be read either. I didn't get reading that property to work. My guess is that channel layout can't be used on the input element, so the fix uses the stream format instead.

The (very old!) information here isn't explicit on this matter:
https://lists.apple.com/archives/coreaudio-api/2004/Jan/msg00139.html
https://lists.apple.com/archives/coreaudio-api/2004/Jan/msg00141.html

We should sort this out. Maybe sending a new mail to the list to sort this out is a good next step.