You're not really giving information that would be helpful. So I'll make
assumptions.

KDE is built against pulseaudio and its api. pipewire, if that is what is
installed on your machine, isn't pulseaudio. As such, KDE will not recognize any
audio device with a specific **exception**: should pipewire-pulse is installed,
it'll serve as a bridge.

I'll reiterate myself, so to get a properly working audio system with pipewire:

1. purge pulseaudio and pipewire related software from your computer
2. install pipewire-audio, *from backports*
3. make sure the pipewire daemon, and friends, are started alongside your KDE
    session.

These tasks should normally be handled by the distro, but pipewire is a bit too
new for debian to figure it out. Also, those tasks would probably be done
through systemd, so we're screwed either way.

Cheers,
                 Ludovic

On Sat, 10 Aug 2024, Adrian Zaugg wrote:

>Thanks to all that replied. I think I was unclear with my description:
>
>- There is sound from VLC, Audacity, MuseScore and others.
>- The problem is that I can't chose an Output in KDE Sound Settings, only
> internal speakers work (but not e.g. headphones on the mini jack):
> The list of output devices is just empty.
>- The KDE volume control is disabled (speaker icon in systray stroked).
>- It has nothing to do with HDMI or not.
>
>To clarify I attach a screenshot of the KDE Audio Settings dialog. There
>should be a list of output devices, but it stays empty. Under inactive
>devices, there is just the "dummy output". To make it more weird,
>after some reboots, this dialog is sometimes populated and everything works as
>expected, but it happens only every twentieth reboot or so.
>
>
>Thanks again for your thoughts!
>
>Regards, Adrian.
>