Here's the article I was thinking about earlier. August 2006 Model Railroader Magazine, page 36. Title---Layouts Should Be Seen and Heard. The author talks about several ways to add sound to the layout itself.
I believe you can order back copies from MR magazine or if you want to give me your mailing address, I will send you a copy of the article. My scanner is not operational or I could do it that way.
Too late now. I've already sent that address to several thousand junk mail sources. Only kidding but is probably would have been a good idea. Maybe you should delete the post. I have your address printed