This happened to me today with a fresh download from x-aviation, the files seem to be under <aircraft>/resources/sounds in various subdirectories.
So for example for me the sounds it is looking for are here:
F:\Program Files (x86)\Steam\steamapps\common\X-Plane 11\Aircraft\X-Aviation\Saab 340A\Cargo Variant\resources\sounds
Incidentally for XP10 they are in the same sub directory under the aircraft dir, resources\sounds and it also fails to load them, installed 26/06/2015, however XP10 doesn't seem to throw the gizmo console up with an error like XP11 did so probably never noticed.
To sort it out copy the contents of the sub directories to the <aircraft>/sounds dir which for the example above is:
F:\Program Files (x86)\Steam\steamapps\common\X-Plane 11\Aircraft\X-Aviation\Saab 340A\Cargo Variant\sounds
So by the time I'm done there are 88 wav files in this dir copied from the resources\sounds sub directories.
I'm guessing some paths got mixed up somewhere either in the installer or scripts.