On my laptop, I have a faulty headphone socket, so when I insert headphones into it, the speakers do not mute. I can confirm that this problem is caused by faulty hardware and not software as when I twist the headphone jack, the speakers come on and off according to the movements.
On previous versions of Ubuntu, I worked around this problem by going into alsamixer and disabling "Auto-Mute Mode", and then going into the sound settings and choosing "Analog Headphones". However, on 12.04, no such option exists, rendering my headphones unusable with no way to work around the problem.
I momentarily thought I had this problem fixed when I installed PulseAudio Volume Control from the Software Centre. I selected the Output Devices tab, and under "Built-in Audio Analogue Stereo" I selected "Headphones" for the port. However, this almost randomly seems to change back to "Speakers", despite me setting "Auto-Mute Mode" as disabled.
Basically, I would like a way to permanently mute the speakers so only the headphones will play sound, without it losing my settings. It is ridiculous that such a simple setting has been taken away to "simplify" the user interface.