We had similar discussions in the past. Stationary under-layout sound misses the auditory experience of moving sound.
To me the sound must be "scaled down" (not very loud) and it has to emanate from the moving model. That is the only way you can experience the sound of the loco passing by your (stereo) ears, and also you can with your ears locate the location of the horn/whistle when when is activated on a loco somewhere on the layout. When the model goes into a tunnel, all the sounds (engine/chuffs/horn/whistle etc.) are realistically muffled. Same goes for brake squeal or rail clacks when a car equipped with sound effects decoder rolls around the layout. All those sounds are directionally picked up by your ears, and to me they realistically simulate the prototype world in a way not possible with stationary sound under the layout. Visualize (or "audiolize") a long train with mid-train or end-train helpers. First you hear the front locos pass by, and as their sound gets weaker you hear the helper's engine sound get louder and louder as it gets close to you. Again, just like in real life.
Also the gut-shaking bass sound of a real diesel engine is IMO totally unrealistic for a 6" model locomotive. It doesn't compute in my brain.
