Control4 does not have the ability to extract audio from HDMI, their control system treats HDMI as a video only source. So to extract audio, you have to trick Control4 by using dummy bindings. Luckily they seem to know this is a problem, and have added dummy audio bindings to most of their drivers. For instance the AppleTV driver from C4 has analog output bindings even though the product does not.
To get C4 to send the proper commands to extract audio, we had to setup dummy bindings along side of the HDMI connections. On the audio side of the Zektor C4 driver there are input bindings labeled HDAUD-xx. These don't actually exist on the back of the switch, they represent the audio extracted from the HDMI inputs.
When you bind HDMI IN-01 to an HDMI source (like AppleTV), you must also bind HDAUD-01 to the same source's audio. Even though there are no physical audio connections being made (only the HDMI cable). This tricks the C4 controller into thinking an audio connection exists. HDMI IN-02 must match HDAUD-02, etc. You can't mix and match, HDAUD-01 must always be bound to the same source as HDMI IN-01, 2 to 2, etc.
Once this is done, Control4 will assume these bindings represent the audio for the room. It will send a command to the driver to switch the HDMI source, and another to switch the audio source. Our driver knows that HDAUD-01 is not a real input, so instead it will switch to the audio being extracted from HDMI input 1. The same for HDAUD-02, etc.