Just a hunch but have you tried to factory reset your TV? Sort of a last resort after triple checking that everything is configured correctly - HDMI cable connected to ARC port, CEC enabled on TV, sound mode set to PCM/stereo, and toggling each of those settings off then back on. Then I'd turn off the TV and unplug it from the wall and let the caps drain, maybe 30s, and turn it back on and re-check.
Having done all that I'd unplug the speakers, do a TV factory reset (maybe check if there's a newer FW update for it too first). If it still doesn't work after all that then it would seem to either be a defect or incompatibility with the TV. Have you tested it on the living room TV?
That sucks though, these seem like perfect bedroom speakers to me, though I really wish it included wifi streaming, especially for the price. If you could stream to them over Airplay they'd be pretty much feature complete.