Have you ever heard of the Air Horn program from Brinston Sound Images?
With Air Horn you use a computer and up to four sound cards to drive speakers located around the layout.
The computer requirements are fairly low so an older tower or desktop will work fine. Pentium II 233mhz minimum. Windows XP operating system.
There is a 30 day trial of the software available for download.
Just one other option.
All Electronics sells that same device for only $2 each. http://www.allelectronics.com Catalog number SAW-3.
How about exactly what you're asking for for 8 dollars?
http://www.bakatronics.com/shop/item.aspx?itemid=690
I've bought stuff from these guys, and they're legit.
It takes an iron man to play with a toy iron horse.
Alan Jones in Sunny Queensland (Oz)
The only thing that comes to mind is a cheap digital voice recorder that maybe you could get to play in a loop. A pretty novel idea I'll give you that. I found this link after doing a quick search maybe this might be the ticket.