So it turned out to be a free/$1 application called SoundBunny. I obtained it from Other World Computing when I bought an SSD. Bunny me.
It allows you to mix the volume outputs of every app/process running on your Mac. Quite handy but I rarely use and at some time in the past must have faded FCP X and PremierPro to zero (probably trying to determine where some unidentified momentary but periodic sound was coming from. It turned out to be the notification bell in Gmail/Google Hangouts and I was binary switching apps volume on/off to trace it). Moral of story: turn down to 1 or 2 not zero you never know when you'll forget to turn back up.