Re: UPDATE: After upgrade to Debian testing (trixie/sid): plasmoids still don't work, KDE doesn't find sound devices.

>  This package contains client libraries allowing programs designed for
>  the ALSA, JACK and PulseAudio APIs to use a PipeWire server for audio
>  playback and recording. They are not used by default, and are currently
>  considered to be experimental.

From what I heard they work fine (for everyday use at least).

>But it depends on pipewire-alsa which, even after selecting it, aptitude
>refuses to install.  It fixes the dependency problem by not installing
>either.

Aptitude does usually (or can) tell you why it won't install something. Have you tried with asking it to install just pipewire-alsa?
Strange that that package isn't installed btw! There are alternatives to ALSA but AFAIK they're older and presumable less advanced.

R