Question: I'd like to have sound output simultaneously from my PC's front and rear jack sockets. 

This used to work fine with Ubuntu 12, but I now have Ubuntu 14.04 and it's one or the other. 

So far I have tried changing the levels in alsamixer in the CLI, no joy. 

However, I notice that in the alsamixer, the entry labelled, 'Headphone' is set to 00 and it cannot be changed, with or without a lead plugged in to the (front) jack socket.

Answer: 
I have discovered the solution, I do not know how this worked:

1. Install gnome-alsamixer (I did it via the CLI).
2. In the gnome-alsamixer GUI, play about with the various mutes and sliders, then set them to what you want them to be.

Both the front and rear audio jacks now output sound (to my PC speakers and my hi-fi speakers). Sweet!

The (other) weird thing is, when I plug the hi-fi (amp lead) into the PC's front audio jack, the PC speakers' volume goes up a bit. If anyone can explain this, I would be interested.
Cheers.
Right - the solution seems to be: using the CLI,start **Alsamixer** (*the Gnome-alsamixer GUI doesn't seem to work for this*) and disable the **Auto-Mute**. Sound comes out from front and rear jacks now. Suh-weetah! I can hear "I Am The Black Gold Of The Sun" by The Rotary Connection in glorious double stereo.