why not use a regular RC system with transmitter/receiver/speed control/battery.
The Main battery can power the loco and the receiver (most receivers have the battery eliminator).
This could fit in a tender or a box/passenger car run behind.
Could be the same set-up as running a scale boat.
You could always add sound modules, just google Ram Sound Modules and there may be something there for you.
A battery set up for a O gauge out door layout would be nice as long as you have a DC motor.