am using one of those USB audio devices plugged into my PS4 (that has 3.5mm mic and headphone ports) and it gets treated as a 'headset' by the PS4. However just by doing a really simple step, I have managed to reproduce this 100% every time to where it will output chat audio over HDMI/optical to an AV receiver or whatever audio device we want to use.
Step 1: Turn on PS4 and connect up one of these USB audio devices (I assume a USB mic will also work the same way, though I haven't tried one yet). Let the PS4 recognize it and assign it to your profile if it asks you.
Step 2: Plug in a pair of headphones to the controller (or any 3.5mm jack, even an empty adapter works, as long as it isn't the headset that comes with the PS4 or anything else that will get recognized as a mic).
Step 3: Unplug the headphones (or whatever 3.5mm connection you used) from the controller.
Step 4: Enjoy chat audio coming from your headphones while being able to use a mic off the USB!
Seriously it is really that simple, and I have been able to make this work 100% of the time. Not sure why it works, but it does.