[ubuntu] XPS 15 with Ubuntu 16.04 and SOUND.

So there's a fairly well documented problem with this laptop and sound issues. Specifically plugging in headphones kills all audio (headphones or otherwise) and only a reboot seems to bring it back.

BUT I've found that if I reboot the computer with the headphones already plugged in I can switch back and forth between exteal speakers and headphones in "sound settings". I just have to keep the
headphones plugged in. If I take them out and plug them back in everything goes quiet again. As far as I can tell everything seems to be working fine when I check, but there simply is NO sound at all that actually
comes through. The various GUI applications I use say that the sound is outputting, but there's nothing until I reboot.

It's almost as if it's a hardware problem, and I would have thought so except both exteal and headphones do work, just so long as I don't unplug the headphones.

Can anyone explain what's going on here?

Thanks.