DIAGNOSIS The HDMI-ARC handshake between the TV and soundbar has failed. The TV audio output setting is often incorrect.
FIX Power cycle both devices. Unplug the soundbar and TV for 30 seconds. Re-plug the TV first, then the soundbar. On your TV, go to Sound Settings and set 'Audio Output' or 'Sound Out' to 'HDMI ARC'. Ensure the soundbar input is set to 'HDMI'.
IF THAT DIDN'T WORK Replace the HDMI cable. Use a certified High-Speed HDMI cable and connect it to the correct HDMI ARC port on your TV (usually labeled).
HEADS UP Some TVs require you to enable HDMI-CEC (often called Simplink for LG TVs) for ARC to function. Check your TV's connection settings.