Let me say up-front that if your issue is exclusively related to controlling the set with your Harmony Hub then I can't be certain you're having the same problems that I am. Also, my tv shares an ARC connection with my receiver which has fully up-to-date HDMI firmware. If you're using an optical-audio connection then we're talking apples and oranges.
In my tv's sound settings "Audio System" mode is selected yet Oreo introduced a couple of new issues with sound that I had never experienced under the previous OS. First, like you, I've had to deal with the set's "mind-of-its-own" when controlling sound output. When powering on the set, which should exclusively use the audio system, it sometimes switches to internal speakers. I always make sure that the receiver is fully started so that it and the tv can complete the HDMI handshake process. Again, this switching to internal speakers never happened prior to Oreo.
Second, when switching from watching TV to using any internal apps (e.g., Netflix, YouTube, MX Player, Spotify) almost 100% of the time I get no sound through my audio system. The only way to hear audio is to switch the tv to internal speakers. It's important to emphasize that when watching TV, which does not use the ARC system because the cable box sends audio directly to the receiver, this problem does not occur. So this only happens when the TV is supposed to send audio from internal apps via ARC to the receiver. Sometimes rebooting the set will restore audio through the receiver but mostly not. Rebooting the receiver has never restored sound.
My conclusion is that these problem were introduced by Oreo and that it's the result of faults in the ARC programming. I posted these issues on the Sony Community Televisions forum and received some guidance from one of the moderators who referred me to a rash of Sony suggested fixes for various audio issues
. You may find one or more of these helpful to solving your issue. For me one in particular
suggests clearing the data and cache of the tv's System app "External Input Services
." I assume this has something to do with ARC because after doing this I can more consistently (90% of the time) switch from TV to internal apps and continue to hear audio through my receiver. I've also noted about a 95% reduction of instances of the TV autonomously switching to its internal speakers on startup.
I hope this is of at least some help. I think we're all waiting for Sony to issue updates that will correct some of the problems introduced with Oreo.
One other small item. After the Oreo update I noticed that the setting (while in an internal app) Action Menu -> Sound adjustments -> Advanced Settings -> Common -> Digital audio out volume
somehow got set to Off or Minimum. Anyone who has updated and is having loss of audio in apps might want to check this setting and move that slider toward, if not all the way to, "MAX."