After searching for ages I found no fix to the problem that meant I could not hear any WAC sound effects apart from the weapons,

Steps:
If you go onto Gmod and start a aircraft and no sound is played, take a look in the console for something along the lines of sound missing from manifest.

If this is the case head over to your GMod directory found at Programs x86, Steam, steamapps, common, GarrysMod, garrysmod: Here you will find the scripts folder delete it and verify gmods integrety by viewing the properties: Whilst hovering your gmod, right click and go to properties.

Hope this helps :D as far as I can tell I am the only one to post a fix on this problem, good luck!

-Shoop