Here's some information that should help:
Bod Backway offers some helpful ideas.
There are several links to suppliers. ITT has pre-recorded sound
modules, MRC has inexpensive sound systems.
I added inexpensive sound to my layout using sound modules and tape
players. I don't use DCC.
Radio Shack and others have recordable sound modules. I purchased
several small tape players with two speakers for $5-$10 each, a
"talking photo album" with ~20 modules from JCPenney on sale for under
$20, inexpensive speakers, etc. found in toys from dollar stores.
It's an interesting project and a challenge to create. Good luck! I
hope your sound experience is as rewarding as mine has been.
Bill's Railroad Empire