I have had increasing problems with my LG TV (UM7500PLA) and Ultra lately. For months I could usually get it to kick into working by switching to optical and back to hdmi, or rebooting everything as a last resort. None of that makes a difference now, and the audio isn't working every single time I start my TV.
I have followed all tips in this forum post and elsewhere and nothing helps currently, except unplugging arc from TV, plugging an audio input to that port, and switching back to the Ultra. This seems to help. But doing it every time I want sound is too much work.
The only other way I can get it to work consistently - which is a little weird - is to load up my Steam Deck and get it to make a sound. For some reason this boots my LG TV into triggering the Ultra, and the sound usually comes on for everything. There must be some way that the Steam Deck formats its audio output that helps, but I can't figure out what.
I have a Nvidia Shield, a Nintendo Switch, and a Virgin Media TV box, all connected over hdmi to the LG TV also, and none of these kick the sound on the Ultra.
It's all starting to get extremely frustrating. And I would say that the more updates the Ultra has got, the worst this LG problem has become.