LG 55LM670T User Manual page 49

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 ARC-supported external audio device is turned on, audio output is
On automatically processed through the external audio device speakers.
For SIMPLINK, refer to 「General → To control external devices」.
✎ Connect to ARC-only terminal when connecting to external devices.
✎ Use high-speed HDMIⓇ cable when connecting to external devices.
✎ Use of a third-party device with an ARC function may cause malfunctions.