I have this problem too with Ubuntu Gnome 16.04 on Dell XPS 15 9550.
All was working fine, then one day it goes silent when plugging in my speakers to the headphone socket.
Looking at pavucontrol and/or "Sound settings" under Gnome it shows that the sound should be playing on the Headphones output port - the VU meter is bouncing up and down ok, but it's silent. Pull out the wire (or select internal speakers for the Output) and the sound switches to the internal speakers OK.
Last time this happened I fiddled for ages and then got distorted sound (sounds like some sort of cross-channel thing) and this only went a away with a complete re-install (which I did to try to fix a separate bug).
I ran alsa-info.sh from the command line and uploaded the info. Here's the URL it gave me to share: http://www.alsa-project.org/db/?f=06...3f7ebb379442a7
It's now happened again, having spent all Friday enjoying music, Monday morning turn on and silence! I've not done any upgrades since.