I can't imagine how you test the audio boards with DC power, you'd get continuous whistle or bell. In any case, they're designed for AC power.
FWIW, I always use a little metal screwdriver for volume adjustments on the RS-Lite boards, I've never had a problem.
To do a basic test, all you need is power to the 2-pin power input and a speaker. That will give you whistle & bell and background sounds for steam or prime mover sounds for diesel.