I've read many, many other posts and not found any solutions that work. I'm a bit of an audio newb and not following all the language.
Using a TCL 55" series 5 QLED, trying to connect to my Denon AVR-S720W receiver.
Connected HDMI cable (tried 2 different cables) to ARC on both TV and receiver. Turned TV speakers off. Tried auto detect, Dolby Digital, Dolby Digital Plus, Dolby Digital DTS. Set CEC to eARC and 1-touch play checked in System.