I used a tiny SBC and a 7.1 channel USB soundcard to upgrade a 15 year old Yamaha AV 5.1 "Dolby Digital" receiver (with toslink/spdif) to handle the latest codecs. The old receiver has a "6ch input" selector that doesn't do any decoding - just takes the 6 inputs, amplifies them and sends them to the speakers. I also use the extra 2 channels from the USB 7.1 audio to a 2nd stereo amp that powers the adjoining room. (This involves pulseaudio channel mapping).
It took about 2 days to make sure that absolutely everything worked, including turning stereo into 5.1 + 2, and adding LFE to the adjoining room speakers so they didn't miss out on multichannel bass. Technically, I could map the extra channels from a 7.1 or 9.1 source into their nearest 5.1 neighbors, but I left that part disabled because I don't have any sources with that many channels.
The most tedious part of the setup was proving that each source type was supported, and all the speaker mappings were correct. Test files are extremely useful and there's no one source for all of them. I spent quite some time finding these files for myself.
I bought a MeLE quiet PC (little bigger than a pi but with case, x86, ssd, 16gb memory) and installed OpenSUSE Tumbleweed because I wanted a rolling distro. Installed kodi, mounted it on the back of a large monitor. Using a cheap Startech 7.1 USB audio on a long USB connection that takes it all the way to the Yamaha receiver. I've attached a 2TB HDD to the MeLE and that holds most of my stuff. There's a NAS in the attic too.
For the pipewire / pulseaudio channel mapping I can go into the boring details if you want
Thanks for the extra detail. I have been wanting to build a dedicated Jellyfin client for a TV for a while to avoid any 'Smart' TV festures. Does your system boot directly into Kodi? I have been wanting to build something similar for a Jellyfin player. Did you set up an IR remote as well?
Well, it boots to a KDE Plasma desktop that launches Kodi (full screen). It also starts MusicBee (windows app that runs perfectly in wine, with a few tricks). I had tried jellyfin but ditched it back when my SBC was a Raspberry pi 4. Because that sucked so much that I abandoned it, attached a large drive and used Kodi instead (ie: I don't stream, I play locally attached content). It's easy to set up a DLNA listener too, so I can fling music or movies to gmedia-resurrect. It's constantly listening in the background, and plays whatever I send to it (eg: from bubble upnp)
Oh, and I plug a Logitech Bluetooth audio receiver into the line-in on the USB 7.1 soundcard, so I can pay music from a phone or my HiBy R2 DAP.
About the Pi: it sucked for a heap of reasons. I found myself fighting stupid battles over and over. Switching to an x86 made all of those problems disappear immediately. Stay the hell away from the Pi, is my advice.
I don't have a remote actually, I use the Kodi Android remote, MusicBee Android remote and kdeconnect (for everything else).
And these channel arrangements can messed up if you pluging headphone/mic in one of the ports, plugin new hdmi device, or for some other reasons. If you use software for mapping surround channels (Virtualmeeter, etc) things can also break. Or channel levels can differ between front and surround channels. Or even if you’re lucky that nothing ever breaks these files are useful during initial fine tuning.
