I am new in Ubuntu, and i want to share you how to fix in some cases this issue.
What you have to do (in this case), is run the terminal in root and type:
"sudo apt-get install paman"
You press enter, and then you reboot
After rebooting, run the “PulseAudio volume control” app, then you choose “output devices” and then in “port” you have to choose “analog headphones”. then you are able to hear in both channels (headphones and loudspeakers).
Sorry for my bad english. Greetings from Argentina