Comment 42 for bug 495134

I've been trying it with the latest upstream code, where it seems like it should work. No auto-switch though.
That is, try installing https://wiki.ubuntu.com/Audio/InstallingLinuxAlsaDriverModules , then reboot. You'll have to select what mic you have manually, e g in gnome-volume-control (try both "Microphone 1" and "Microphone 2") or alsamixer (Input source should be set to "Front mic" - the external mic is labeled just "Mic").

Report back whether it worked or not. Thanks!