Let's see - that has a "portable input" jack, probably 3.5mm stereo, and the "red and white" jacks on the back are input ports.
http://www.lg.com/uk/home-cinema-systems/lg-DH3120S-home-cinema-system
The primary audio outputs for the PS4 are audio over the HDMI connection, but you aren't using HDMI, and the optical connection, not present on your home cinema machine. But wait: There is a headphone output. This is most likely 3.5mm stereo, so you can attach the headphone output to the "portable input" on your home cinema.
"You can set the audio that is output to the device such as a headset. You can output all audio, or output only chat audio. Select [Sound and Screen] > [Audio Output Settings] > [Output to Headphones]. The default setting is [Chat Audio]."