After months of working fine, out of the blue, the sound card of my laptop would not show up anymore in Unity Control Center. Any application that would produce sound would hang if asked to play (vlc, rhythmbox, mpg321, etc...).
The sound card still did show up when I was running the command:
I rebooted multiple times to no vail. I search the web for a solution and found this web site How To Fix: No Sound In Ubuntu 14.04 And Linux Mint 17.
I applied the first suggestion which is to run:
sudo alsa force-reload
and then reboot (required). After these operations, the sound card showed up and everything worked again.
Now I have no clue why suddenly that happened. Fortunately for me, I am computer savvy and knew how to search for a solution on the web (and fortunately for me, it was easy, taking me 15 minutes though to find it).
For a normal consumer, the only solution would have been to reinstall the OS, or another one.