Sorry to raise an old thread from the dead but after days of hassle and manual editing of config files and swapping between pipewire & pulse audio and back again I can confirm that DD 5.1 live encoding issues (i.e. missing sink so it cannot be chosen) has absolutely nothing to do with the kernel version you are on.
It’s all down to ALSA config. It does work fine out of the box in some situations on EndevourOS (which is quite frankly amazing no other Distro does that I’ve EVER tried) but then a system update trashed my config. It does not work ‘out of the box’ in a virtual machine with GFX card passed through (nvidia 3080), but doing what’s below enabled 5.1 AC3 support over HDMI
The ‘exact package’ = alsa-plugins (installed by default) which contains the A52 encoder (also called AC3/DD Live/Dolby Digital Live 5.1) for ALSA but then weird config file shenanigans are also required to get it to actually work.
how to get AC3 over HDMI working in pipewire properly…
‘pactl list sinks’ to find out your card number, use it in the ‘asound.conf’ and ‘.asoundrc’ files below.
Then run the following commands:-
sudo touch /etc/asound.conf
sudo nano /etc/asound.conf
Then insert the following into /etc/asound.conf:-
pcm.a52 {
slavepcm “hdmi:CARD=0,DEV=0”
}
Then save the file. Now do the following:-
cd ~
touch .asoundrc
nano .soundrc
Then insert the following into .asoundrc:-
cm.!default {
type plug:surroundaudio
}
ctl.!default {
type plug:surroundaudio
}
pcm.a52encode {
type a52
format S16_LE
channels 6
rate 48000
bitrate 448
}
pcm.surroundaudio a52encode
ctl.surroundaudio {
type hw
card 0
device 1
}
Then save your .asoundrc file.
Finally reboot or restart the pipewire service and the AC3 output option will now be available in pavcontrol / plasma audio settings 
There is a lot of mis-information saying you need to fart around with FFMPEG due to a depreciated library that A52 depends on for the alsa A52 plugin (which is installed by default on EndevourOS but not enabled in certain situations, that’s what the asound configs are for), that info is old and now completely duff. It’s also said that pipewire does not support AC3/A52 encoding, this is also complete rubbish too.
Hope this helps anyone else trying to get Dolby Digital Live 5.1 over any HDMI port working.
Thingi