LG 55LA6600-SA.KWGYLWK User Manual page 37

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
automatically processed through the external audio device's speakers.
On
For SimpLink help, refer to 「User Guide → OPTION → 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.