Changing the fading distance of sounds

[Roblol99]

Does anyone know how to change the distance for which sounds in the game fade as the player moves away from the source. ive been able to change some sounds to whatever i want and change the volume in the LUA files but i havnt found where you can change the fading distance. i know there is a audible distance modifier but it doesnt seem to be related to the sounds i want to change, like the train sounds. can anyone help?