LG 84LM9600 User Manual page 31

Even when the connected ARC-supported external audio device is
Off turned on, audio output is not processed through the external audio
device's speakers.
When an ARC-supported external audio device is turned on, audio
output is automatically processed through the external audio device's
On
speakers. SIMPLINK must be turned on.
For SIMPLINK, refer to 「General → To use input device」.
✎ 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.