Created attachment 129236 [details] pacmd list-cards output On my laptop (Lenovo ThinkPad X1 Yoga), audio devices are represented as different devices, not as multiple ports of the same device. So for example I have two microphone devices: one for the audio jack and one for the built-in microphone array. The plasma-pa applet shows both, and gives each one a combobox so yu can switch between them. Alas, it doesn't work. Clicking the radio button for the headphone device doesn't work, and audio is always recorded from the built-in microphone array. There is also a menu item saying "record all audio via this device" but that also does nothing.
Created attachment 129237 [details] How plasma-pa applet looks for me
It's possible that this is ultimately a PulseAudio or kernel problem, since the microphones really should be combined into multiple ports of the same device. For that, I've filed https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/issues/913 and https://bugzilla.kernel.org/show_bug.cgi?id=208133 (not sure which level of the stack is at fault for that).
Found out that this is not an issue in the applet. The audio source for the jack is always disabled/unplugged due to driver bugs, which is why switching to it doesn't work. I will report a PA or kernel bug.