I suppose a video tutorial is called for, since explaining the process is in words is complex and cumbersome, but i'll give it a shot.
You cannot simply add a motor sound byte into the train folder and expect to hear it, obviously. You need to go into the train.dat file and manipulate the code found at the "end" of the file (motor sound codes take up the majority of the Train.dat file). In addition, you need to enter the sound.cfg file and make sure the motor sound you just added is coded for.
When I edit motor sounds for my trains, I use the train editor application, since it allows you to manipulate the motor sound code without going into the train.dat file and playing with the actual numbers. You will see in the tutorial video. Give me a few hours ok?