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Just to update you all on this:
Thanks Bojcha. We were successful in getting it to work but even with the volume in Windows an Stereo Tool set as high as possible, coming out from the back L or R jack sockets of the Behringer UMC202HD, using a straight TS to BNC or a TS to RCA Photo (with a Phono to BNC adapter), the max deviation was 50kHz (with an average of somewhere in the 40s), so it was very under-modulated, to the point where one of our car radios wouldn't even lock to the frequency for more than a few seconds and tried to seek. The best solution for the moment seemed to be to use the UMC's headphone out at exactly 50% volume (knob at 12 o'clock) with the MPX volume in Stereo Tool at ~0.70dB. By the way, I dumped the Ubuntu 20.04 idea and just installed Windows 10 21H1.
A Focusrite Scarlett 2i2 2nd Gen was also tested with the same results as the UMC coming out from the rear L or R outputs.
If anyone has any experience with either of these soundcards going to a transmitter, did you use the headphone jack also, or did you manage to get it correctly modulating coming out from one of the rear outs?
First, Soundcard(s) are not problem there, problem is transmitter. It should have MPX level adjustment so rise it up.
Second, 50kHz is not so much under-modulated at all, and locking to station when you seek on receiver has nothing to do with it. That's again up to transmitter or Receiver (bad or picky) or signal reception.
Never use headphone outputs except for headphones.
So, double check in windows (Control panel > Sound) and/or Soundcard software is Master maxed out to max. Search for MPX Level adj on transmitter and rise it up.