Stan, I did try the experiment with an 8 ohm resistor, the range was still affected the same when a sound was playing. I suspect you're right about the RF circuit, it doesn't have the components that I'd expect for a superhet receiver.
I wasn't suggesting taking advantage of the stereo in the rolling stock, that's why I figured one channel would be available for the control function.
I took a look at the signals coming out of one of the DAC channels, they look like normal audio. If they're generated with PWM, they have added filtering before the outputs. I went pretty high frequency and didn't see evidence of full voltage excursions that I'd expect with PWM. Here's a few samples of my test sounds at the left DAC output, I assume the right would be similar.
Coal loading into tender
Reefer Compressor
Track Running Sounds