The Audio Unit and its callbacks run in a completely separate real-time thread from any other thread in you app. So any commands given to an Audio Unit should be assumed to be executed asynchronously. And possibly not until many tens of milliseconds after a command is issued.To be safe, in my apps, I never free any related resources until several seconds after an audio unit is told to stop. And I never stop an audio unit if I plan to immediately (re)start that same unit. I just leave the audio units running, passing silence (zeros) in all the callbacks.