I am new to this forum, so I hope this thread is still getting some attention.
I was also looking for a way to hook up my headphone + mic combo (akg k712pro+anlion modmic) with the PS4 without having to buy the Astro Mixamp.
My setup is: optical cable from the PS4 to my FiiO-Dock (E17+E09k) where my headphone is connected to. And the mic is plugged into an USB soundcard (CSL) wich is plugged right into the PS4 USB port.
The headphone output of the soundcard is empty.
At first this setup didnt work out for me, as the PS4 has recognized the soundcard as a "headphone" device. I could adjust the volume of the mic but couldnt hear myself talking.
!!! NOW by accident (or maybe it was just frustration) i removed the mic from the usb card, hooked it up into the dualshock controller same thing, nothing happening.
>>>> BUT after removing the mic from the controller and plugging it back into the soundcard I was able to hear myself talking. The first thing I did,I jumped right into an online game to check if the chat works, and it does. I could hear other players and they could hear me talking. <<<
So as you can see, it is possible to get the gamesound AND chat through optical, while you're still able to talk through usb.
I hope somebody of you can confirm this, and see if it works.