The PS4 has the ability to output both game and chat audio via the USB port. Since the LS30/LS31 receives the game audio via the optical cable, the setting on the PS4 need to be changed from “All Audio” to “Chat Audio” so that only chat audio will come through the USB port.
On the PS4 press and hold the Playstation button on the controller. Scroll down to Adjust Sound and Devices. Select Output to Headphones and change the setting to Chat Audio.