I was referred to this company by forum member scratchbuiler1-48. I love their products. They make sound modules to place around the layout. I have another thread going that has the bridge and highway crossing sounds with their 4 inch 8 ohm speaker. They are very affordable. I just received 5 more modules today after ordering them Wednesday night so their service is awesome. The modules run on AC or DC , can be connected to loop the sounds or play once and have a volume adjustment. We plan to have sound behind our waterfall, city traffic sounds in the city, fire trucks by a fire and also some shop sounds by some maintenance buildings. If you haven't given them a look I highly recommend them
Here is the website http://ittproducts.com/