I have 2 microphone always available one is from the webcam and the other one from the audio card.
I have at every boot the webcam microphone as default also if I use the KDE audio panel to set the other one or also with pavucontrol.
I don't know how to force it at every boot, I was thinking to remove from drivers the support of microphone from the webcam but I don't think that is the best solution.