A few days ago, Ubuntu 14.04 64-bit started to show a bug: occasionally, when I resume the machine from hibernation (sleep/suspend mode), it came back okay, except sound was not working, was muted, no audio. Quite puzzling, as a search for
"Ubuntu 14.04 Audio mute after resume from suspend / hibernation"
"Ubuntu 14.04 restart no sound"
brought no similar sounding issues others were reporting.
Anyway, one thread mentioned how to restart ALSA, and trying that was a success: audio was back. Anything else, muting/unmuting audio in GUI, moving the volume slider etc. was not successful. So do this to restart alsa and recover sound after hibernation in case it's muted:
$ sudo alsa force-reload