Thanks, Yampa Bob for the specific reply.
I assume that the Tsunami decoder will replace my existing factory-installed decoder, right?
Are there add-on solutions that just add sound to the existing decoder?

I would be interested in the part numbers for the speaker and the snap-on baffle you mentioned.  This is very new to me -- I've never even taken the shell off the locomotive -- so any relevant information/help offered is appreciated.

What is the best way to upgrade the GP40 locomotive sold with the E-Z Command Sets for sound?