Changing Points/Junction Sounds

[CityRailTrainspotter]

I'm a real beginner with editing content. What I want to do is change the sound when a train goes over a point/junction. I have sound files I created but I have no idea how to change the default sounds. As I said, I'm a beginner with this sort of stuff so I hope I don't get confused
I've read this one:
http://forums.auran.com/trainz/showthread.php?58743-Enough-of-the-junction-sound

But I got confused at the 3rd post...

Any help appreciated