Spruce is right about the GP9 ...
As far as I just checked, only the .proxyxml of GP9 and F7 are refering to the "engine" folder, instead of "cab" folder as per instructions, so only for GP9 and F7 you should (for now) drop the wav files into the "engine" folder, ie:
\railworks\Assets\Kuju\RailSimulatorUS\Audio\RailVehicles\Diesel\F7
\railworks\Assets\RSC\GP9Pack01\Audio\RailVehicles\Diesel\GP9
However, for the .proxyxml you must keep following the instructions, ie drop them into:
\railworks\Assets\Kuju\RailSimulatorUS\Audio\RailVehicles\Diesel\F7\Cab
\railworks\Assets\RSC\GP9Pack01\Audio\RailVehicles\Diesel\GP9\Cab
I'm sure Brian will check this, and update both .proxyxml, or review the instructions.
Cheers,
Doc.
Who doesn't have dog, hunts with cat.