Manjaro KDE - After the last update replaces pulseaudio with pipewire, no more audio volume controls

Do you have pipewire-media-session installed?
Also check

systemctl --user status pipewire-media-session.service

And enable and start if not active (running).

I discovered that it is needed to install and activate pulse-pa.

But this is in case you already switched to pipewire. I did and everything is fine for me now.

This topic may be useful.