** This workaround did work for me for a while – I have now had to implement a new workaround to fix the issue now that it had resurfaced again. See this post for details. **
Having just installed Ubuntu 12.04 (Precise Pangolin) on my Dell Vostro 3750 I noticed that there was no sound when I plugged in my headphones.
The workaround was to edit /usr/share/pulseaudio/alsa-mixer/paths/analog-output-headphones.conf.
Update: Since implementing this workaround I have found that updates have reverted the changes that I made to analog-output-headphones.conf. So I recommend making a backup of the original file, then making changes to analog-output-headphones.conf and then backing up the changed file.
Open the terminal and enter the following commands:
cd /usr/share/pulseaudio/alsa-mixer/paths/ sudo cp analog-output-headphones.conf analog-output-headphones.bak sudo nano analog-output-headphones.conf
Look for the section called [Element Speaker] and change it so that it looks like this:
[Element Speaker] switch = on volume = ignore
Create a backup of the corrected analog-output-headphones.conf:
sudo cp analog-output-headphones.conf analog-output-headphones.fixed
Save the changes and then reboot.
After rebooting I found that I needed to remove my headphones and then insert the jack again to get them to work. Once they were working I could remove and insert the headphones jack and the behaviour was as expected.
With my configuration files backed up I can now easily revert to the original settings should I need to in the future or copy the fixed settings back if the settings are lost.
Credit to Ryan (bovorasr) for the workaround.