I wish Lionel had the ability to change sound files like MTH. Then this would be easy/free.
I've had some issues with the whistle or bell sound MTH has used in their files. I just use this program to correct it: http://www.silogic.com/trains/ADPCM.html
No having to buy new sound boards. Change the whistle or bell to something you like better and upload it to the locomotive. Done.