In the past, the MC-EX has been flaky with sounds. I took to using either a standard R2LC or one of the many RCDR receiver boards that Lionel has available, those have proper serial data.
I'm starting a "deluxe" version of an R2LC motherboard, you can see it here.
Smart Locomotive R2LC Motherboard Project
The nice thing here is you can populate as little as you need , depending on the application. If you only need the basics, just one or two connectors and the R2LC pins would do the trick, I already used a couple of my prototypes in exactly that way. It's actually way more than the MC-EX, but it'll obviously work in a lot more scenarios.
