Henry, let me know what you did when you are finished. I am thinking of trying the same thing for my railroad.
The amplifier IC arrived today and I've breadboarded the circuit, but it's not working yet.
I'll have another try tomorrow. I think the 9V battery is a bit low on charge, but didn't have my meter handy to test it.
When I get it going, I know that I can switch it on and off with an Arduino pin through a transistor to produce the 'chuffs' and relate that to the speed of the motor. Long, slow 'chuffs' as it starts to move, faster, shorter 'chuffs' at full speed. What I've no idea how to do is cross-fading between two or more speakers. I'm thinking of using PWM, but I'll leave that until I have one speaker working properly.