half-solved
I did it just now (realtime-privileges and realtime group membership).
That removed the RealTime error message at application startup.
But, it still makes only crackly sounds just the same. Also tried reboot and remove-reinstall. So, I’m presuming there is more to be done with the audio stack, probably with PipeWire, but I don’t want to guess.
The message itself says it wants “RealTime priority… with ALSA and Jack”. So, probably that isn’t happening, even with proper realtime permissions.