At the risk of jumping in where angels fear to tread...
I cannot see that there is any technical reason why a sound generator cannot be designed that could accept, say, pairs of audio tones (similar to tone dialling on a phone) and, combined with an A to D converter, automatically produce the sound of a train starting up, accelerating and running at speed. The operator would have to initiate the process manually. With 3 frequencies one could have 3 commands (A+B, A+C and B+C). Four tones would give 6 commands, etc. Some commands would have to be paired to give on/off (diesel engine sound/steam loco sound) while others would be stand-alone (e.g. whistle/horn, venting steam). The same frequencies coudl be used for all locos on a layout as the signals would be limited to the loco being driven by a particular controller.
However, I cannot see it ever happening.
All the best,
PS added as an afterthought. Could one actually generate the sounds in the controller and superimpose them onto the DC voltage. All the loco woudl need then is a small speaker to make the appropriate noises. (There would have to be at least a capacitor in series with it to prevent dc reaching the speaker.) Would relatively low level audio signals have any effect on the motor?