Issues of surround sound transcoding for Chromecast

Surround sound is transcoded by default if there isn't AC3/E-AC3 passthrough chosen. When streaming starts it's transcoded to multichannel Vorbis codec. If it's a 5.1ch sound it's encoded to 6ch Vorbis when streaming starts which Chromecast decodes to the 5.1ch PCM audio and everything is fine.

But if you seek through video or change stream configuration e.g. you change the audio track, switch off the video track, etc, a subsequent Chromecast stream is then MPEG Layer-3 encoded and decoded to a 2ch Stereo PCM. When you stop (not only pause) the video and play it again, streaming starts from the beginning and thus the correct again. The issue comes if you need to change the audio track because you lose setup and cannot set it up again while the video is stopped - workaround is to start a video with the local renderer, change an audio track and then change the renderer to Chromecast.

Playing video with AC3/E-AC3 passthrough is without such an issue.

Another issue is with a 7.1ch audio and more channel audio probably as well, because it's transcoded to an 8ch Vorbis, but Chromecast instead of decoding it to the 7.1ch PCM produces just the 2ch Stereo PCM.

With AC3/E-AC3 passthrough, audio is played as surround sound well, just with a minor defect - it's signaled as a 5.1ch E-AC3 on receiver input instead of the 7.1ch...

What BTW brought me to the idea that AC3 as well as other streams like e.g. DTS have frame structure with distinguishable sync words so it could be possible that a receiver recognizes the right codec even though it would be improperly CEA-861 signaled over HDMI as E-AC3 because of Chromecast Surround sound passthrough implementation is like in The Peasant's Wise Daughter (neither naked nor clothed, neither walking nor riding, neither on the road nor off it) allowing this way passthrough of other codecs. I think it could be worth of experimental feature... ;-)

Setup VLC 3.0.17.4 Vetenari for 64-bit Windows Chromecast 2, Preview Program, System firmware 275994, streaming device firmware 1.56275994 AV Receiver Pioneer, capable of up to 8ch PCM 192kHz/24b, DD, DD+, DTS, DTS-HD, D-THD...

Note - the issue reported in #26736 likely has the same root as the first issue.