Hi!
Since today noon I've that problem - and I had not changed anything. Today morning everything has worked as it should.
AS shows the correct speaker list, plugin 1.1.9, AirFoil 4.9.1, Indigo 6.1.4, Mac OS 10.11. Disabling and deleting the plugin's prefs didn't make any change, also tried several reboots of the whole system. The plugin always reports "No speakers available".
Cheers,
Ben