Comment 8 for bug 978880

Ok, I have a workaround that works for me:

sudo vi /usr/share/pulseaudio/alsa-mixer/paths/analog-output-headphones.conf

Now, change the Element Speaker section to this:

[Element Speaker]
switch = on
volume = ignore

now, run alsamixer, and ensure that the AutoMute option is enabled.

What this does is it changes the sound profile pulse audio switches to when headphones are inserted to a profile that *does not* turn down the speaker slider volume. The AutoMute option ensures that the external speakers stop playing (if it were disabled, the headphones would work, but the external speakers would keep playing).