Okay, now "pactl list short sinks" lists:
1 alsa_output.platform-soc_audio.analog-stereo module-alsa-card.c s16le 2ch 44100Hz RUNNINGI can play on this from my computer but it‘s just stereo like the name suggests (strangely, it isn’t "analog-stereo", since I get it over HDMI audio). On native Kodi UI though, I can play 5.1 material in 5.1 (no passthrough used, just PCM). So, the 5.1 option is still missing in PulseAudio‘s exposed interfaces.
Hey, thank you. But of course did I run the whole chain of commands:
LibreELEC:~ # systemctl stop kodi
LibreELEC:~ # pactl load-module module-udev-detect
16
LibreELEC:~ # pactl list short sinks
0 auto_null module-null-sink.c s16le 2ch 44100Hz SUSPENDEDBefore and after, I can choose between four audio devices in LibreELEC‘s settings: "PI: HDMI", "PI: Analogue", "PI: HDMI and Analogue", "PULSE: Default, Bluetooth Audio (PULSEAUDIO)". It makes no difference what I choose, even before the commands above, "pactl list short sinks" still shows me only "auto_null".
It seems I‘m not the only one with this problem.
Hello,
I want to use my RPi2 as 5.1 output device on my AV receiver and want to be able to send audio from my Arch Linux computer running PulseAudio in 5.1 to the surround system. The 5.1 output on the RPi seems to be unfinished but with LibreELEC’s applied patches I get full surround sound playing videos with the UI, nice! Now, I want to leverage this hardware support (had no luck with Xubuntu and none with Raspbian) to use the PulseAudio networking feature, good documented in the LibreELEC wiki.
But here comes the problem: "pactl list short sinks" lists only this:
0 auto_null module-null-sink.c s16le 2ch 44100Hz RUNNING
And guess what, it’s really a null device, I checked. 
So, any way to get the 5.1 output established to PulseAudio? Thank you!