DIAGNOSIS: HDMI-CEC and ARC handshaking issues are super common. The TV isn't recognizing the soundbar as an audio output device, so it defaults to its own speakers.
FIX: Power cycle the whole chain in order. Turn everything off. Unplug the soundbar and TV from power. Unplug the HDMI cable from both ends. Wait 2 minutes. Plug the TV back in, turn it on. Go to Settings > Sound > Audio Output and make sure it's set to "HDMI ARC." Now plug in and turn on the soundbar. Finally, reconnect the HDMI cable between the TV's HDMI 3 (ARC/eARC) port and the soundbar's ARC port. The TV should now detect it.
IF THAT DIDN'T WORK: Try a different, high-speed HDMI cable that's certified for ARC. Also, go into the TV's CEC settings (Settings > System > Control other devices (CEC)) and toggle "System audio control" off and back on. This can force a fresh handshake.
HEADS UP: If you're using other CEC devices (like a game console), they can sometimes confuse the ARC handshake. Try disconnecting other HDMI devices temporarily to see if the soundbar starts working, then reconnect them one by one.