Umm… possibly dumb suggestion. Try restarting your computer? Maybe that’ll free up the audio engine. But take this with a huge chunk of salt as even I haven’t installed Dorico 4 yet so I have almost no idea what I’m talking about. // Edit: also I just stumbled across another audio engine issue where EuCon was crashing it. Maybe this will fix your issue? Dorico 4 hangs on startup - #4 by richhickey