Is this going to be an "always on" sound, or one that you trigger with some on-layout event (like a train passing) or a manual button, to run for a short time?
You might consider just getting a recording on disk (or making one yourself) and playing it on a cheap CD player.
I picked up a few of these:
http://www.allelectronics.com/make-a-store/item/SAW-3/SAW-3-DIGITAL-VOICE-RECORDER/1.html
It's a 30-second recording which you can provide. I haven't gotten to it yet, but I plan to put in a "flange squeal" sound and trigger it with a magnetic reed switch when the subway trains approach the station. Not sure about the sound quality, but with sound engines running anyway I figure it will work well enough, and you can't beat the price.
It takes an iron man to play with a toy iron horse.
I don't think I know what the Walthers HO scale sound system is. But there is a company that advertises in MR called ittproducts that sells digital sound modules. Their website is: http://www.ittproducts.com/hqpage_2008.htm. Item HQ520 will give you ocean, seagulls, and foghorn. If you scroll down to this item on the link and click on the speaker icon, you'll get a sound sample.
I don't have any of their products so I can't vouch for what they sell. The provided link is for information only.
Does anyone know of a good fog horn sound system for Walthers HO scale sound system? I was able to find this one but not sure if this is what I am looking for. Thanks in advance.
http://www.horizonhobby.com/Products/Default.aspx?ProdID=RAM63