I have the same loss of audio on my lg cx oled tv with a denon avr-x3700h. Both units are brand new. All of my hdmi cables are new high speed . Everyone having this issue including me seems to be able to fix it with a hard reset “ unplugging “ the components. This fix is only temporary the problem comes back sometimes in a day or maybe it takes a couple of weeks. I believe there is a hardware or firmware bug with the hdmi ARC and eARC boards in these components. I know this doesn't help anyone out but this is what I believe is happening. Im going to contact the manufacturer of the components . If I have any luck ill post it here.