I have a Dell XPS 13 9350 on which I have installed Linux Mint 18.1 Cinnamon.  Everything is working perfectly, even the Dell Dock WD15, except that after some time the sound devices disappear from the sound app.  Here's what the sound app looks like right after booting:

after a little time passes, while I'm just using the system or the system sits idle, the sound app looks like this:

If I go into a terminal, not as root, and execute this command:

pulseaudio --kill

the sound icon first disappears from the panel, then after a few seconds it reappears and once again the sound app sees the devices.  Again, after some idle time, or working time, the sound app will display no sound devices.

Any thoughts?

Thanks!