Default sounds are always top-level in your sounds directory...
Custom sounds for individual planes will only take effect if the directory of the same name as the model is present...
Yes.. Best to .wav
If you want detailed-detailed specs about the sounds... You don't use the in-game controls... Open your Aces High folder, find a sound file you wish to inspect, right-click, and properties...
And last but not least... Check6 sound file is specific to every model... Which is merely copied to every model directory... And in turn not really a default sound...