MacOS has sometimes troubles with audio.
You play something and ends in a loop.
Your Mac was asleep, you play something and wont play properly
You have clicks and glitches
Your interface stuck, after connecting it.
Something is not picked
and more.
Command "KILLAUDIO" will "reset" all audio drivers, which are automatically restarted after some seconds. It simply kills all drivers, easy but effective.
http://www.packal.org/workflow/audio-killer