Hmm… I just got around to applying the stable updates from 2024-05-13 onwards, and having spent a good while following the guide to dealing with the latest system Python updgrade.
I’m not 100% sure, but I think it’s this latest (2024-05-29) update which includes Pipewire that has caused my problem. My laptop speakers work fine, but my wired headphones do not work at all. I’m using wireplumber as a session manager.
I think someone else had a similar problem with the first big update ([Stable Update] 2024-05-13 - Kernels, Plasma 6, Gnome 46, LxQT 2.0, Nvidia, Pacman - #259 by DrRevert) but the fix (involving ~/.config/wireplumber) is not relevant to me.