At present, when “System Audio” is set as the source in Airfoil, the “Computer” output is not available. Because System Audio capture relies on the Windows WASAPI Loopback device, it's impossible to filter out local playback from the final audio stream. Playing to the local output would therefore result in a feedback loop. To avoid this, local playback is disabled when System Audio is selected as the source.
In order to send audio to both remote outputs and the local “Computer” output, choose a specific application or device source in the Source selector.