Trains are incredibly loud and noisy contraptions. I'm imagining a scenario where sound modules can be put in EVERY car - not just in the expensive engine. This only can happen if cost gets low enough...but that's exactly what has happened in the last couple years with these insanely-priced, miniature-sized Asia electronic modules. MP3 players, RF remote modules, etc.
A byproduct of the Fitbit and smartphone activity trackers and navigation systems are these motion detection modules for less than $2 shipped (121658851348). 3-axis accelerometer and 3-axis gyroscope on a single IC chip. It's absolutely incredible considering just a few years ago you'd pay a fortune to just get a single-axis solid-state accelerometer chip.
Yes, it requires a microcontroller or processor and lots of software to take advantage, but there are countless O gauge applications if someone puts their mind to it. Let's see. Detecting going around a curve to trigger wheel squealing, detecting initial motion or stopping to trigger coupler slacking or compression, detecting rail joints to generate truly synchronized clickety-clacking, well you get the idea!
To date it's been impractically expensive to put sound in EVERY car (plus the engine) and maybe it's a fool's errand to do so. But imagine the effect of a consist rolling by that actually sounds like the prototype...rather than sound coming only from the engine and 50 trailing cars silently tagging along...
Also, in my opinion, O gauge is the smallest scale where the sounds can have enough volume and fidelity to make it worth the effort. Yes, speakers have gotten better over time and to each his own, but I think it takes about a 50mm/2" speaker which just fits into O and will never fit in HO or S.