I did try the experiment with an 8 ohm resistor, the range was still affected the same when a sound was playing.
Geez. You're probably thinking I've sent you snipe hunting 
. Well, maybe time to give the direct-RF remote a rest and focus on the TMCC version since that's what you need. If you do come back to it, there's more snipe hunts available to the hunting enthusiast - conducted susceptibility via the power or i/o lines.
I remain convinced that a low-cost remote "system" to activate rolling-stock anywhere on the layout is something O-gauge could really use.
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.
That's pretty slick if they integrated the filtering on the chip and/or module. As I recall, some microcontroller chips that feature "analog" output pins are BYOF (bring-your-own-filter). Of course for this hack of using the spare audio track as a control channel, I'd rather have the digital pulses but that's tomorrow's problem. Thanks for taking those scope shots, I can see the 1/2 scale DC offset on the DAC channels.
As you may recall, I'm fiddling with the barebones $1 MP3 module plus external amplifier module. I'm finding each O gauge application has some quirk to work out. For example, in the case of servo PWM, to get the pulse timing resolution to smoooothly rotate the mechanism, let's say you want 1 part in 100 resolution. Since the pulse width varies about 1 millisec for full servo swing range, that means a pulse resolution is 10 microsec. Back-of-envelope says that requires a sample rate of at least 100 kHz. Well, for example, I see your MP3 module specs a max sample rate of 48 kHz...and I'm sure my MP3 module is no better. So then you start playing games...or as the saying goes all ideas eventually degenerate into work. I'm just learning about how the MP3 compression algorithm and these low-cost MP3 chips handle DC-coupled digital pulse streams.
