It's been answered here before, but yes, it's by design. There are some scenarios where the sounds will play at a low volume when interior, however. Outdoor sound effects are also enabled if you're in an interior view, the cabin door or cockpit is open, and the engines are off. Carenado may be using some custom datarefs or even active sounds that are canceling this out in this scenario. We're only at version 1.0, so we'll definitely look into it.