Hi Steve,
the Beier sound module is great, expensive and requires a fair bit of programming. A far easier set up is the sound module from Harbor Models. it comes ready to install, plays the engine sound and a horn without one sound interrupting the other. On Harbor Models website you can listen to which engine sound suits your model best, order that version and when you receive it just plug it in and that's it.
There is also a sound module avialable from Mtronics. It comes with a speaker. I have installed it in my Portgarth tug but wished I had gone for the Harbor Models unit instead - the large diesel sound is much better (in my opionion).
Whatever you decide to buy - a model with sounds is MUCH more interesting than a model lacking this feature. I think ;-)
Best
Claes