For those who lose sound via HDMI ARC after a firmware update, CEC/ARC has been buggy on many sets for a long time because of complex digital crosstalk between different manufacturers devices being involved. We will all be thankful when we get to the eARC 2.1 world.
The reason it would manifest after a firmware update is that the HDMI handshake can get out of whack when the TV power is interrupted for the reboot. Almost by accident, I discovered that if you use this exact procedure and sequence you should be able to get fresh HDMI handshake:
--turn off the CEC or HDMI control on each and every external device that connects to your tv or hub via HDMI (i.e., Ultra Blu ray player, cable/sat box, FireTV Stick 4k, Chromecast Ultra, ROKU Premiere, Apple TV 4k, etc.).
--turn off the CEC or HDMI Control in whatever serves as the "hub" of your system (AVR or soundbar). First ensure the AVR or soundbar's own volume is set at audible levels.
--turn off the ARC and CEC in the TV settings, and leave it at TV speakers. Set the TV speakers volume down to zero.
--unplug the hdmi cable from tv Input 1.
--power everything off.
--power on the hub (avr or soundbar) ONLY, not the tv yet or anything else
--plug the hdmi cable back into TV Input 1
--enable CEC or hdmi control on the hub
--power on the TV and ensure the Dolby Digital Plus is enabled
--Test audio using an internal app such as Youtube or Pandora. From now on, leave the TV volume at zero and control your volume using only your hub.
--Enable the CEC or HDMI control on each external device connected by hdmi.
The odd thing is that I now get audio including Atmos over DD+ via HDMI Input 1 even though my setting in the TV is still TV speakers (but I know ARC must be active) and the TV CEC is completely off. No more volume slider pop ups so it works for me. However, I totally agree that if HiSense is listening, they need to fix this bug more permanently in the next update if they want happy customers.
As for those who can't get Atmos out via HDMI 1 from the internal Netflix app, nobody can. It is not a Hisense H9F flaw. You can't get Atmos from the Netflix app in the Fire TV 4K stick either, or any Netflix app in a device for that matter with the sole exception of the Netflix app in Apple TV 4K (and some high-end Samsung TV Netflix apps). Evidently Apple and Netflix have an exclusive deal there (and things like this explain why Apple TV 4K is a relatively expensive streaming device).
Overall, I still like the H9F and will be keeping it--picture quality is just excellent, period. I am a big Sony fan and never thought I would have a HiSense, but it is a great thing that we will never again have to pay over $1000 for a 65-inch TV with 120 hz panel, FALD, quantum dot and Dolby Vision again, so HiSense has done us consumers a real service for now. Can't wait for the mini-led/dual panel 75-inch 8Ks from TCL and others in a year or two!