So, I've settled on a combination of sounds: the DTM GP50 engine sounds for the exterior, and the DTG GP40-2 for the cab sounds. The exterior sounds work fine, but there aren't any cab sounds, except for some of the switches and levers clicking, but no engine sounds.
I tried copy/pasting the XMLs from the required DLC into the proper folders and tweaking the file paths in the audio BIN files to match, no dice. Then I tried outright overwriting the BIN files in the hopes that something in the original files might be the key to success, but still no cab engine noise. As a last resort, I went into the engine BIN file for the Sherman Hill SD40-2 (the one I'm trying to mod) and changed the file path so that it referenced the requested engine and cab audio files, that just changed the sounds back to the default Kuju sounds for some reason.
I'm completely stumped as to why nothing seems to be working.
