I just discovered a stupid bug. Apart from my HT, I connected my two bookshelf speakers to my TV via 3.5mm jack. Whenever I open Netflix with 3.5mm plugged in, even though audio is set to ARC, Netflix doesn't enable 5.1 but once I remove the jack, it enables the 5.1. It's freaking weird. May be make sure you connected just the optical.
I did spend some time today to learn more about audio codecs.
What I did is, I downloaded various 5.1 format test files encoded with different codecs such as,
A52 Audio aka AC3 a52 (Dolby Digital)
DTS Audio dts
ec-3 (Dolby Digital Plus)
FLAC Free Lossless Audio Codec flac
MPEG AAC Audio mp4a
PCM S16 LE s16l
TrueHD Audio trhd
And tried playing them in Plex app (via plex server running on Raspberry Pi) as well Kodi (directly from the NAS) and here are the results,
In all case for Plex,
Passthrough and Optical Encodings for AC3 and DTS are enabled.
And for Kodi,
Passthrough and AC3, E-AC3, DTS and DTS-HD enabled.
A52 Audio aka AC3 a52 (Dolby Digital)
Played correctly in plex app without needing transcoding.
Played correctly in Kodi.
TV Audio Digital Audio Out set to Dolby Digital (Passthrough option works too)
DTS Audio dts
Played correctly in plex app without needing transcoding.
Played correctly in Kodi.
TV Audio Digital Audio Out set to Passthrough
ec-3 (Dolby Digital Plus)
Played correctly in plex app with transcoding (EAC3 - AC3)
Played correctly in Kodi.
TV Audio Digital Audio Out set to Dolby Digital Plus (Passthrough option works too)
FLAC Free Lossless Audio Codec flac
Plex did not show the file at all
Kodi did not show the file at all
MPEG AAC Audio mp4a
Played correctly in plex app with transcoding (AAC - AC3)
Kodi did not play 5.1 correctly.
TV Audio Digital Audio Out set to Passthrough
PCM S16 LE s16l
Plex did not show the file at all
Kodi did not show the file at all
TrueHD Audio trhd
Played correctly in plex app with transcoding (TRUEHD - AC3)
Kodi did not play 5.1 correctly.
TV Audio Digital Audio Out set to Passthrough.
---------------------------------------------------------------
So what I understand is, codecs which TV natively cannot understand, my Plex media server (raspberry pi) transcoded and hence it played correctly with plex app.
And Kodi support AC3, E-AC3, DTS and DTS-HD and
NOT AAC and TRUEHD, hence it could not play files MPEG AAC Audio mp4a (My original test file) and TrueHD Audio trhd correctly.
Whatever codec it don't understand, it simply throws the sound to all speakers together.
With Primevideo (I only have prime), There is no way I can check what codec it use for 5.1.
It probably something TV cannot natively support, and hence it will not work correctly.
It may very well be working as well, but since no test audio available in prime app its hard to confirm. Simply sound comes from all speakers means its not true 5.1 I believe.
So in this case, only an ARC supported receiver may be the only option (Which will decode 5.1 audio correctly and send to speakers)