When multiple AIRA Compact units that are connected via USB to the same computer are mutually connected to the MIX IN/OUT jacks, a humming noise may occur due to a ground loop. This noise occurs in certain situations when there are multiple circuit paths to the GND (ground), and is not an issue that’s particular to the AIRA Compact. Check whether the noise still occurs when the units are not connected to the MIX IN/OUT jacks.
If the purpose of connecting AIRA Compact units to your computer’s USB ports is solely to supply power, connect only one unit via USB, disconnect the others, and use a USB AC adapter to power them instead.