LG 55LA6205 User Manual page 33

Even when the connected ARC-supported external audio device is
Off turned on, audio output is not automatically processed through the
external audio device's speakers.
If an ARC-supported external audio device is turned on, audio output is
On automatically processed through the external audio device's speakers.
For SimpLink help, refer to 「User Guide → To use input device」.
✎ Only use ports that are labeled ARC when connecting to external devices.
✎ Use a high-speed HDMIⓇ cable when connecting to external devices.
✎ Use of a third-party device with an ARC function may cause malfunctions.