Hi there, long time Home Assistant user, first time poster.
I’ve benefitted from a few people’s blog posts on Home Assistant recently and wanted to contribute something back, so I wrote up how I used Home Assistant to trigger all of my home theater hardware to turn on via a single (light) switch.
The post is at Automating a Home Theater with Home Assistant for anyone interested.
Thanks for the share and nice automated theater 
A suggestion of improvement I have done with mine in living is to setup automatically lights at proper dim values when playing starts on my media player (Emby) and toggle lights at higher values when you go into pause.
I did something like this using virtual toggles (switches) in HA. One for “Backyard movies”, one for “Bedroom 1 Entertainment” and another for “Living Room Entertainment”. I take it a step further because with the exception of the source devices, which is a NVDIA Shield TV (Pro), the outlets to run the audio and TVs are shut off to reduce vampire draw as these devices are off.
Since I use Plex for pretty much all my entertainment, when Plex starts playing, it adjusts the lights in the room as well as any other settings I deem necessary and then when it stops or pauses for more than 10 seconds, it brings everything back from where it was before the content started.
Depending on your setup, you could do things like adjust your thermostat, close window shades, fans, lights, door locks, turn certain cameras on or off, etc.
For example, when my patio theater is going on, my backyard and side door cameras motion detections are disabled as well as recording and the back door lock is prevented from being engaged. Since the air conditioner is back there and it is assumed the house is virtually empty except for me going in and out for snacks, beverages and restroom, then when on air conditioner, the temp of the house is risen up to run the A/C outside less.
Love the idea of having the lights die down when a movie starts. I set up a Jellyfin instance recently so I guess it probably has similar capabilities - some kind of web hook or similar that HA can subscribe to to trigger workflows when things like playback starts/stops?
Can you please show me an example of how you are restoring settings after 10 seconds?
For me this is specifically lighting brightness.
I’m a HA n00b, and this is my first try at doing any real automation, and I keep hitting brick walls.
Thanks!
David Buttrick