I found t
his diy project.
Then there is
this video on installing a sound module in a diesel locomotive.
On Aliexpress I found
this inexpensive module.
There is also the option to
place the sound unit in a passenger car if there is insufficient space inside the locomotive. If space is an issue, you may also consider placing the sound module and the speaker in different units, e.g. the sound module in the first passenger car and only the speaker in the locomotive. There is a drawback, however: locomotive and passenger car need to be connected by a very flexible wire and cannot easily be separated.
A huge selection of sounds specific to the type of locomotive matched to different decoders and speakers is
here (for Diesel scroll down the page).
