I had thought about that external speaker solution too but I don't know, I can't imagine it working as well as on board sound to be honest. The way I imagined it working was a fade in/out at "strategic locations of interest" so you'd mount a speaker at every station, yard, level crossing, signal etc. and just fade the appropriate audio in as the train approaches the speaker position, then fade the sound out as it moves away. No cacophony of sound from all these speakers. The trains run silent through "open country".
For the baby GMs you could just buy Loksounds rather than trying tos squeeze the Bluetooth boards inside. A hybrid solution. Cheapo boards where they fit, Loksounds where major surgery is required.