I have an antenna hooked up to my Tivo, TV Tuner, and HD Homerun.
Tivo and ATV are connected directly to my receiver. TV audio is connected via ARC - my TV is 5 years old so doesn't have eARC
When I watch shows on Channels DVR on ATV, I get 5.1 as expected, exactly the same as I do on my Tivo or my TV Tuner. I get no 5.1 sound on when using the HD Homerun ap on ATV. But I do get 5.1 sound on the HD Homerun ap on my Roku, which is plugged in directly to my receiver.
I did hook my ATV up to my tv, and then back to my receiver via ARC. When using the HD Homerun ap, i DID get 5.1 sound, but it was on some channels which did not have 5.1 sound on my Tivo or TV (and these were sub-channels, which I know do not have 5.1). So I think via ARC my receiver was encoding the signal and incorrectly playing 5.1. Thankfully I don't have a way to add atmos speakers. Not sure how I would even do that with this set up
Long story short, I think the HD Homerun ap for ATV is messed up.
As mentioned earlier, I get 5.1 fine from Channels, except when I turn on PIP. If i exit the Channels ap with PIP, when I come back to the ap (via closing PIP), I no longer have any 5.1, and have to restart the Channels ap.