Why is he kidding? It'd be fun to add sound to one of those. Kids love sound effects
I thought for a long time about how to add sound to my homemade thomas, which is made out of a wooden pull toy. But I could not figure out where to put the speaker and sound card. It's made out of mostly fiberboard/mdf which does not "hollow out" all that well.
I'd use a small scale railways sound card--they are very small and inexpensive. The first sample on their sound sample page sounds pretty good to me
Sierra Soundtraxx (IF you Can find them) offers a nice "Peanut Whistle-Euro-Logging" card that would be a nice match....
Small Scale cards are nice and most folks really like the ease of install and use. http://www.smallscalerailway.com/01-02.html
try this one:Bavarian S3/6 BR18.4 (European) 4-6-2 it's the last one on the list!
If this is for the kids, I'd get my hands on one of those 'Scientific Toys" train sets, often sold at KBToys or ToysRUs. They only cost $25-ish, and you get a battery-powered loco with a NOISY sound system that chuffs and whistles. Kids love them.
Hopefully, you can figure out how to chop it apart and fit the bits into a Lionel Thomas. Otherwise, when the Lionel Thomas chassis dies, as I've heard they do, just transfer the body to the Sci Toys chassis and remove the pilot.
You could also try an LGB European Steam unit. Self-contained and easy to hook up (so I've read here). Hard to synch the chuff unless you have LGB MTS software, though. Every once in a while you'll see one on eBay (or a "Sound Tender" which I assume contains the same unit?).