Additional information: the problem with audio is also present at gdm, before logging in. So it’s not a user problem.
September update:
I made another experiment: I reinstalled EOS Gnome from scratch.
The problem is present at the very first boot (both at gdn and after logging).
The status of wireplumber says:
❯ systemctl status --user wireplumber
● wireplumber.service - Multimedia Service Session Manager
Loaded: loaded (/usr/lib/systemd/user/wireplumber.service; enabled; preset: enabled)
Active: active (running) since Fri 2024-09-06 18:13:56 CEST; 2min 4s ago
set 06 18:13:56 lg-eos-kde systemd[828]: Started Multimedia Service Session Manager.
set 06 18:13:56 lg-eos-kde wireplumber[933]: spa.bluez5: BlueZ system service is not available
set 06 18:13:56 lg-eos-kde wireplumber[933]: wp-device: SPA handle 'api.libcamera.enum.manager' could not be loaded; is it installed?
set 06 18:13:56 lg-eos-kde wireplumber[933]: s-monitors-libcamera: PipeWire's libcamera SPA plugin is missing or broken. Some camera types may not be supported.
So I tried to install
sudo pacman -S pipewire-libcamera
Rebooted and the sound immediately works! Both at gdm and after logging in!
I shut down the computer and boot it again and guess what: the problem is there again… 
By the way, instead of muting and unmuting, another way of bringing the sound to work is to restart wireplumber
systemctl restart --user wireplumber
But for sure, Gnome has a severe problem with pirewire/wireplumber, I would say…
at least, after reinstalling, hibernate works again, but probably that’s unrelated and it’s only a kernel upgrade.