I think that the crux of the matter may be that .... where steam is concerned there are fewer and fewer people who can recall (and therefore imagine) the real thing as it used to work ... including the sounds. Heritage lines give some idea - but of course they don't have proper goods trains, proper shunting, and all the wrong locomotives are pulling the wrong trains etc, etc.
It could be argued that in some ways it is through good, prototypically operated models that we get some kind of window through into aspects of a lost world and as such sound (even if just as a background) might have a place. Certainly when I was last at Pendon I learnt a lot from the operators when viewing the dartmoor scene ... but for me it would have been enhanced by the addition of sounds! In the end it might only be suitable for top end layouts that can tap into specific expertise, but I am still watching developments with interest.
The thought of a realistic backing track to the train movements on Little Bytham would be a mouth watering prospect