Thanks for your input Richard. I agree there is definitely effort in the 'program' however, whilst I'm no expert in programming locomotive sound files, the diagram you provided appears to be a fairly basic logic flow for when to play what sound. I work with far more complicated logic flows (I've worked in IT for 30+ years).
My point isn't that these people shouldn't be compensated, it's that the effort involved isn't vast by any stretch of the imagination, and that effort is compensated for by a percentage of each sale. On that basis I still can't see the justification for the addition unit cost of adding sound to the decoders. It honestly seems very excessive to me.
Happy to be pointed in the right direction, but none of what I've read so far appears to justify the cost.
Thanks everyone for contributing to the debate.